[Thorsten Loesch]

i doubt theres any audible difference between lithium batteries and supercaps (unless the gryphon is capable of more than its battery allows that is which i doubt as thats what it was designed for)

The Gryphon is based on tech I generated in 2559 (Thai Date, love it, even the ancient past is way ahead of everyone else's today) or so, which is about seven years ago. Back then it was among the best I could do on the original budget (199 USD intended retail for the original xDSD).

I have since had many occasions to experiment with super capacitors (and other things) and I find that all of the power supply elements have their own uses. It is all about relative discharge (and charge) speeds. Super Capacitors are great for situations that need high peak power quickly, like for example car drive trains, or indeed audio.

We must understand that music is not a steady state waveform as used in testing using test gear. Music looks like this.

The light blue colour is the average power, dark blue the peak in the music waveform (upper graph).

In the lower graph we see spectrum and we can see that low frequencies have the most energy.

So power drawn from the supply varies a lot with music and has high peaks at low frequencies relative to a (1kHz) steady state test signal.

In power supplies we use different parts for different frequency ranges or device works at.

Batteries work best with a near constant current draw. The same is incidentally true for "off mains" power supplies, both linear and switching.

Capacitors can provide "extra" current dynamically for peaks, but charge held in capacitors is relatively small next to batteries for the same size.

The small ceramic capacitors we place close to IC's are very "fast". We are talking near ideal operation up to tens of MHz, but they cannot store much energy. So at low frequencies (in the audio range) they become ineffective with a given signal current. Hence they are good for supporting power supply lines for individual digital IC's.

Electrolytic capacitors which we use on power supply lines are much "slower". Usually they can only operate well up to 10kHz or so, some "exotic" types are good up to a few 100kHz. But they can store much more energy, so they remain effective to much lower frequencies. normal values in portable gear tend to be ok down to perhaps 100Hz or so, it depends.

For really low frequencies their size becomes too small.

In desktop or big hifi products we can simply enlarge the values, making everything physically larger (and heavier and more expensive). In portable gear this is an issue, as usually we cannot.

So audio peak current needs to be supplied by the battery. The problem is that the chemistry in batteries is pretty lazy. So the battery is slow to ramp up current.

Meanwhile the step-up power supply that supplies the whole device (3.0V-4.2V battery to 5V or 5.7V for audio) will try to draw more current, you get an avalanche condition where peak power is limited by the slow battery chemistry and the battery voltage is being drawn lower and lower, until the battery shuts down.

So we need something that kind of "slots in" into the gap between moderate size electrolytic capacitors and battery or off mains supply.

Super Capacitors happen to be the answer. Of course, implementation is slightly non-trivial. You cannot just put them in parallel with a battery.

Here an actual illustration of what adding suitable super capacitors can do to peak output with music:

True, this is relevant only with difficult to drive headphones or say a portable bluetooth speaker etc.

Certainly, if I were to design an audio product today, it would incorporate super capacitors. Had I designed the Gryphon, not only would it have skipped a few extra op-amp stages that are functionally redundant and result from copy/paste merging two stand alone products, but it would have had super capacitors (and no OLED screen).

But I didn't have a hand in that product, other than having designed the products it was derived/copied from. So it is what it is.

Thor

 

[pioterd]

Super Capacitors are great for situations that need high peak power quickly, like for example car drive trains, or indeed audio.

Thank you Mr Loesch. If it is like this, why super capacitors were not introduced earlier, or today they are so slowly incorporated into products? They are mostly beneficial for the lower bass? Require physical space?
ZENCan has 4x1000uF capacitors onboard. Would it be beneficial sound-wise if there were 4x2000uF?
Another thing is that negative-feedback amplifiers. Is the Gryphon similar in that regard to NFCA circuits used by Topping?

 

[Thorsten Loesch]

If it is like this, why super capacitors were not introduced earlier, or today they are so slowly incorporated into products?

Only recently do we see low cost super capacitors with low ESR, that are useful in the kind of circuits we use in Audio.

The other side, you cannot simply "stick one in", many available supercapacitors are 2.7V rated and need extra circuitry to reliably be connected for higher voltages.

So overall implementing supercapacitors in products is still in early stages.m

They are mostly beneficial for the lower bass? Require physical space?

Not in this sense. They allow more output power at low frequencies. The physical space vs. actual value is favourable.

For example, a 470,000uF/5.5V with 0.3 Ohm ESR capacitor can be as small 16 X 8 X 14mm.

It could easily be fitted in portable gear with 5V rails.

ZENCan has 4x1000uF capacitors onboard. Would it be beneficial sound-wise if there were 4x2000uF?

The external power supply is the main limiting factor here. Going from 1,000uF to 2,000uF will have minimal effect, try 4 x 10,000uF instead. The actual capacitor quality has a large impact, more than the value. The "Signature" versions use Panasonic Os-Con instead of good quality generics.

Another thing is that negative-feedback amplifiers. Is the Gryphon similar in that regard to NFCA circuits used by Topping?

I did not work on the Gryphon, I can only guess stuff from looking at the circuit board.

There are many styles of "Nested Feedback" Amplifier designs, I'd credit Ed Cherry and his NDF system from 1982 as the originator. Many variations of such "compound" circuits are possible.

I implemented my take in what is being sold as Diablo and Neo. I think the Gryphon got the Neo "power amp" but I could be wrong.

While these nested systems can deliver lower measured distortion than systems without such, this does not mean that they necessarily or reliably sound better.

Thor

 

[reynaldohm]

@Thorsten Loesch

For iFi Audio NEO iDSD I bought iFi iPower Elite 5v.
I thought it turned out excellent.

 

[lantian]

Only recently do we see low cost super capacitors with low ESR, that are useful in the kind of circuits we use in Audio.

The other side, you cannot simply "stick one in", many available supercapacitors are 2.7V rated and need extra circuitry to reliably be connected for higher voltages.

So overall implementing supercapacitors in products is still in early stages.m



Not in this sense. They allow more output power at low frequencies. The physical space vs. actual value is favourable.

For example, a 470,000uF/5.5V with 0.3 Ohm ESR capacitor can be as small 16 X 8 X 14mm.

It could easily be fitted in portable gear with 5V rails.



The external power supply is the main limiting factor here. Going from 1,000uF to 2,000uF will have minimal effect, try 4 x 10,000uF instead. The actual capacitor quality has a large impact, more than the value. The "Signature" versions use Panasonic Os-Con instead of good quality generics.



I did not work on the Gryphon, I can only guess stuff from looking at the circuit board.

There are many styles of "Nested Feedback" Amplifier designs, I'd credit Ed Cherry and his NDF system from 1982 as the originator. Many variations of such "compound" circuits are possible.

I implemented my take in what is being sold as Diablo and Neo. I think the Gryphon got the Neo "power amp" but I could be wrong.

While these nested systems can deliver lower measured distortion than systems without such, this does not mean that they necessarily or reliably sound better.

Thor

One question to you would be what's your favourite price no object capacitors for audio power supplies? Mine so far are audio note kasei or mundorfs ag mlytics. But have been wondering about quality of polymer electrolytic caps for some time now, are they comparable with the best wet electrolytic's purpose made for audio or perhaps are even better, in sonic effects on the whole system?

 

[emba4]

The Gryphon is based on tech I generated in 2559 (Thai Date, love it, even the ancient past is way ahead of everyone else's today) or so, which is about seven years ago. Back then it was among the best I could do on the original budget (199 USD intended retail for the original xDSD).

I have since had many occasions to experiment with super capacitors (and other things) and I find that all of the power supply elements have their own uses. It is all about relative discharge (and charge) speeds. Super Capacitors are great for situations that need high peak power quickly, like for example car drive trains, or indeed audio.

We must understand that music is not a steady state waveform as used in testing using test gear. Music looks like this.



The light blue colour is the average power, dark blue the peak in the music waveform (upper graph).

In the lower graph we see spectrum and we can see that low frequencies have the most energy.

So power drawn from the supply varies a lot with music and has high peaks at low frequencies relative to a (1kHz) steady state test signal.

In power supplies we use different parts for different frequency ranges or device works at.

Batteries work best with a near constant current draw. The same is incidentally true for "off mains" power supplies, both linear and switching.

Capacitors can provide "extra" current dynamically for peaks, but charge held in capacitors is relatively small next to batteries for the same size.

The small ceramic capacitors we place close to IC's are very "fast". We are talking near ideal operation up to tens of MHz, but they cannot store much energy. So at low frequencies (in the audio range) they become ineffective with a given signal current. Hence they are good for supporting power supply lines for individual digital IC's.

Electrolytic capacitors which we use on power supply lines are much "slower". Usually they can only operate well up to 10kHz or so, some "exotic" types are good up to a few 100kHz. But they can store much more energy, so they remain effective to much lower frequencies. normal values in portable gear tend to be ok down to perhaps 100Hz or so, it depends.

For really low frequencies their size becomes too small.

In desktop or big hifi products we can simply enlarge the values, making everything physically larger (and heavier and more expensive). In portable gear this is an issue, as usually we cannot.

So audio peak current needs to be supplied by the battery. The problem is that the chemistry in batteries is pretty lazy. So the battery is slow to ramp up current.

Meanwhile the step-up power supply that supplies the whole device (3.0V-4.2V battery to 5V or 5.7V for audio) will try to draw more current, you get an avalanche condition where peak power is limited by the slow battery chemistry and the battery voltage is being drawn lower and lower, until the battery shuts down.

So we need something that kind of "slots in" into the gap between moderate size electrolytic capacitors and battery or off mains supply.

Super Capacitors happen to be the answer. Of course, implementation is slightly non-trivial. You cannot just put them in parallel with a battery.

Here an actual illustration of what adding suitable super capacitors can do to peak output with music:



True, this is relevant only with difficult to drive headphones or say a portable bluetooth speaker etc.



Certainly, if I were to design an audio product today, it would incorporate super capacitors. Had I designed the Gryphon, not only would it have skipped a few extra op-amp stages that are functionally redundant and result from copy/paste merging two stand alone products, but it would have had super capacitors (and no OLED screen).

But I didn't have a hand in that product, other than having designed the products it was derived/copied from. So it is what it is.

Thor

what an absolutely beautiful reply thank you. maybe you should design an audio product now, id certainly be interested to see/hear the results

 

[Thorsten Loesch]

One question to you would be what's your favourite price no object capacitors for audio power supplies?

That is one of these apparently simple questions, that only has very complex answers.

Or a near useless simple one, like:

"Whatever has the correct electrical parameters to deliver the performance I require"

which is technically absolutely correct and the indisputable, but also completely useless to anyone but me, myself and I.

So instead perhaps I will explain "what I do" without justifications or claims, just what it is.

Recently I got a killer deal on a Marantz PM-75. It is a 2532 model year integrated AMP with more than 100W into 8 Ohm per channel in Class AA (aka non-swiching class AB) and a TDA1541 based DAC that is 95% of a CD/DA-12 combo on board.

I ended up paying 300 USD for a 95% mint unit, Japan standard with a 1kW 220V to 100V transformers, untouched and not refurbished minor scratches on top cover and back.

Nice condition for a 34 year old unit, definitely non smoker and careful owner(s). Defo way chuffed. One of the crazy sleepers...

Obviously, it's overdue for a redo.

I am pushed for time right now, so parts arrived but are still on the shelf.

The Op-Amp's are a mix of NJM5534 (power amp), NJM5532, NJM2068 and NJM4560. The circuit generally is very 'pro audio', I like it and I will not "mess around", but restore and improve in the original context.

Capacitors are a mix between half decent and "What", I guess they had to save money somewhere.

Here is what I ordered, to refurb this Amp. I order from Mouser, so if they don't stock, it's not on my list.

Main PSU Capacitors (15,000uF @ 80V) will get changed to Nichicon Muse "Super Through" 18,000uF @ 100V (IIRC) which have the same diameter and fit existing mounting hardware but are much taller, barely clearing the casework. These account for over a third of the total redo parts bill, which comes in at more than I paid for the unit, a lot more actually. Without my work, it will be a ~ 1,000 USD DAC/AMP when finished.

All other power supply capacitors, except on the digital Boards are Nichicon Muse KZ. Wherever physically possible, larger value. Digital power supplies use Panasonic Os-Con.

All Op-Amp's are to be replaced by TI parts with FET input and circuits refactored to take advantage of this.

I got OPA828 for the power amp, OPA1656 for anything else. Suitable DIP8 adapters.
As a result most coupling capacitors in the signal path will be switched to WIMA FKP02. A number of mylar and ceramic signal circuit capacitors will be replaced with Xicon brand Polystyrene types.
What remains as signal circuit and needs values that cannot be shifted to FKP or MKP from WIMA are Nichicon Muse ES "Supergreen" bipolar types.
I also ordered self adhesive copper tape, to improvise a ground plane on a pair of single layer digital PCBs that predate modern technology.
As the physical sizes of IC's and other parts are hopeless by modern standards, but at the same time the highest frequency in circuit is ~ 1.6Mhz I ordered Panasonic SMD film capacitors to replace every single digital bypass capacitor.

I would use SMD ceramics in a scratch design, a mix of of 0402 1nF C0G and 1uF X6S being generous with these caps, they make grains of rice look huge, so we can.

As said, anything larger than 0.1uF is Panasonic Os-Con, usually oversized by a factor 3-10... Several digital rails will get 1,000uF Os-Con instead of something like 47uF or 100uF.

That's it.

I have many most excellent reasons for each choice, but they either need a post grad lesson in electronics plus physio and psychoacoustics for a semester to really get down to it, or they fall under "Whatever has the correct electrical parameters to deliver the performance I require"...

It's what I'd use if I where to build or rebuild a "near cost no objective" item. I'm not convinced that more expense for truly exotic parts would be justfied. But I am happy to receive free samples to change my mind. Hehehe.

I hope this answers some questions and is of use, if perhaps not what was expected.

Thor

 

[lantian]

That is one of these apparently simple questions, that only has very complex answers.

Or a near useless simple one, like:

"Whatever has the correct electrical parameters to deliver the performance I require"

which is technically absolutely correct and the indisputable, but also completely useless to anyone but me, myself and I.

So instead perhaps I will explain "what I do" without justifications or claims, just what it is.

Recently I got a killer deal on a Marantz PM-75. It is a 2532 model year integrated AMP with more than 100W into 8 Ohm per channel in Class AA (aka non-swiching class AB) and a TDA1541 based DAC that is 95% of a CD/DA-12 combo on board.

I ended up paying 300 USD for a 95% mint unit, Japan standard with a 1kW 220V to 100V transformers, untouched and not refurbished minor scratches on top cover and back.

Nice condition for a 34 year old unit, definitely non smoker and careful owner(s). Defo way chuffed. One of the crazy sleepers...

Obviously, it's overdue for a redo.

I am pushed for time right now, so parts arrived but are still on the shelf.

The Op-Amp's are a mix of NJM5534 (power amp), NJM5532, NJM2068 and NJM4560. The circuit generally is very 'pro audio', I like it and I will not "mess around", but restore and improve in the original context.

Capacitors are a mix between half decent and "What", I guess they had to save money somewhere.

Here is what I ordered, to refurb this Amp. I order from Mouser, so if they don't stock, it's not on my list.

Main PSU Capacitors (15,000uF @ 80V) will get changed to Nichicon Muse "Super Through" 18,000uF @ 100V (IIRC) which have the same diameter and fit existing mounting hardware but are much taller, barely clearing the casework. These account for over a third of the total redo parts bill, which comes in at more than I paid for the unit, a lot more actually. Without my work, it will be a ~ 1,000 USD DAC/AMP when finished.

All other power supply capacitors, except on the digital Boards are Nichicon Muse KZ. Wherever physically possible, larger value. Digital power supplies use Panasonic Os-Con.

All Op-Amp's are to be replaced by TI parts with FET input and circuits refactored to take advantage of this.

I got OPA828 for the power amp, OPA1656 for anything else. Suitable DIP8 adapters.
As a result most coupling capacitors in the signal path will be switched to WIMA FKP02. A number of mylar and ceramic signal circuit capacitors will be replaced with Xicon brand Polystyrene types.
What remains as signal circuit and needs values that cannot be shifted to FKP or MKP from WIMA are Nichicon Muse ES "Supergreen" bipolar types.
I also ordered self adhesive copper tape, to improvise a ground plane on a pair of single layer digital PCBs that predate modern technology.
As the physical sizes of IC's and other parts are hopeless by modern standards, but at the same time the highest frequency in circuit is ~ 1.6Mhz I ordered Panasonic SMD film capacitors to replace every single digital bypass capacitor.

I would use SMD ceramics in a scratch design, a mix of of 0402 1nF C0G and 1uF X6S being generous with these caps, they make grains of rice look huge, so we can.

As said, anything larger than 0.1uF is Panasonic Os-Con, usually oversized by a factor 3-10... Several digital rails will get 1,000uF Os-Con instead of something like 47uF or 100uF.

That's it.

I have many most excellent reasons for each choice, but they either need a post grad lesson in electronics plus physio and psychoacoustics for a semester to really get down to it, or they fall under "Whatever has the correct electrical parameters to deliver the performance I require"...

It's what I'd use if I where to build or rebuild a "near cost no objective" item. I'm not convinced that more expense for truly exotic parts would be justfied. But I am happy to receive free samples to change my mind. Hehehe.

I hope this answers some questions and is of use, if perhaps not what was expected.

Thor

Thanks, actually super similar to what I'm in the process of doing to my luxman l190, it's 9184model with 30wpch at 8ohms.
Only you actually have real understanding of how these things are designed or actually work, I'm just replacing components with mostly equivalent parts that I find most transparent and musical.
So far have found increasing all electrolytic cap sizes to basically 2x very beneficial, but also atleast in my luxman not to mess with the capacitor value in nfb loop(messes up sound )
Mouser is a bit of pain in the ass to deal with in Germany, atleast where I live I have to be home to get the package otherwise they drive around with it and send it back after a while.
So order most parts from hifi collective UK
So far found audio note kasei, bypassed with appropriate fkp or mkp caps are my favourite, use mundorfs cause could get them locally in my city, very pleased with their performance, could not say I have heard any weaknesses.
How about resistors, are you going to leave them stock?
Imagine you will replace those aswell.
Almost got the pm80 one for it's ability to go true class a bias, but working on my luxman have been to much fun and had way to good of results to stop untill have done it all.
Wish you a fun time with your Marantz and best of success with first try.

 

[emba4]

ive often dabbled with the idea of getting an older or cheaper amp or even amp kit then replacing as many parts as possible with high quality ones to see the results, EG can you turn an average amp into a good one by upgrading its components or will it just make a mess of the sound? ive been looking at some of the Kevin Gilmore dynahi boards/kits for just this reason. as with the user above i usually buy my caps, resistors etc from hificollective, mouser/digikey/RS are great for selection but their websites are a bit of a nightmare to navigate

 

[Thorsten Loesch]

So far have found increasing all electrolytic cap sizes to basically 2x very beneficial

I am not saying increasing capacitance never helps, but double is usually not really that significant.

In case of the Zen CAN, these capacitors follow a regulated switching converter that switches at 1.2MHz and thus theoretically can provide regulation up to around 300kHz, the actual regulation bandwidth is more like 120kHz, so in the audio range the voltage is mostly regulated by a fast regulator. So the capacitors serve more to stabilise the feedback loop of the switching regulator and remove switching noise, than actually to support audio signal current.

Mouser is a bit of pain in the ass to deal with in Germany,

In Thailand dealing small foreign suppliers is a huge headache. Mouser get you to prepay duty and tax and just send the stuff, yes, someone need to be at the (home) office to receive the delivery but they stick to normal Office hours.

How about resistors, are you going to leave them stock?

Resistors seem carbon fibre. I ordered several engineering kit of 1/4W, 1/2W & 1W nichrome metal film resistors from Aliexpress, from a factory I know, so they are known quality. I will change those that matter.

Wish you a fun time with your Marantz and best of success with first try.

Thank you, it will be quite a job.

Thor

 

[Thorsten Loesch]

ive often dabbled with the idea of getting an older or cheaper amp or even amp kit then replacing as many parts as possible with high quality ones to see the results,

Many of the older HiFi gear from the "Japanese Golden Age" are actually extremely high quality. It is usually the upper end items. There is an excellent website dedicated to this, called "The Vintage Knob":

http://www.thevintageknob.org/marantz-PM-95.html

The PM-95 is one step up from the PM-75 I have here.

Another good site is the Audiodatabase:

https://audio-database.com/MARANTZ/amp/pm-75-e.html

If you take 77,000 Japanese Yen in 1989 - that's 1,400 USD in 2023. This is not a cheap item.

If you were to restore such a unit and modify the old style 120 Ohm headphone out to a more sensible modern impedance and level, you have a DAC that makes CD sound better than most modern DAC's manage with real High Res recordings and a hugely powerful Amplifier than can drive nearly 30V RMS into any headphone and uses a "non-switching Class AB" design that sonically is much closer to Class A.

If you like the kind of sound their products make, you could buy a modern DAC and a 500W/8Ohm Amp from that Company and pay over 11,000 USD and I am not certain that the result will be preferably to a fully restored PM-75, with CD (and vinyl as there is a really good phono stage as well). Of course, there is a lot of stuff modern products do that the PM-75 cannot, like DSD, HiRes, wireless audio etc., while the PM-75 has good and useful tone controls.

Of course, there are also many rather average or poor pieces from that era. Sturgeon's law* applies.

Not every "Marantz PM-xx" Amplifier is great. Some are even rather poor.

EG can you turn an average amp into a good one by upgrading its components or will it just make a mess of the sound?

If you take a very high end, aspirational piece of Japanese made "High End" hifi from 30-40 years ago, you can get a Product that performs absolutely excellently, without any qualification needed. It's true High End. The Sony ES series are usually good, as are the top end Marantz/Philips items and there others. Luxman had many excellent items in their upper tier products.

Now electrolytic capacitors especially do not age well and need replacing best every 10 years or so. Another factor, if the Product uses Op-Amp's they are invariably poorer than what we have access to nowadays.

So IF you accept the limitations (e.g. the TDA1541 Single Crown Chip DAC in the PM-75 is 32/44.1/48kHz 16 Bit only, no HiRes, no DSD, no DSD 512, nothing, just CD standard) you get very high quality items that need refurbishment.

But you can often buy such an item second quite cheaply. Look around.

Of course, if you start with a T.U.R.D (that a technical term, Technically Utterly Ridiculous Device) at the end you still have a T.U.R.D.

If you start with might be considered solid gold Item that has aged poorly and you restore it (polish it, buff out scratches etc.) you will have solid gold that looks amazing, or in the case of HiFi great sound.

But no, a poor or average amplifier will not magically turn into serious high end with expensive parts.

ive been looking at some of the Kevin Gilmore dynahi boards/kits for just this reason. as with the user above i usually buy my caps, resistors etc from hificollective, mouser/digikey/RS are great for selection but their websites are a bit of a nightmare to navigate

The Dynahi uses parts that are obsolete and hard to get. Most of the parts you will be able to buy will likely be fakes. If you have all original parts, it should be pretty good, there are a number of areas where the circuit could be improved. And the power supply will be quite important. I'm a great believer that the power supply should be designed together with the signal circuits and need to work well together.

I would buffer the VAS Stage with folded emitter followers (aka diamond) and convert the input stage to the Lender circuit. This significantly improves linearity. Seeing the J-Fet's are obsolete, it may be better to use bipolar transistors in the differential amplifiers and add J-Fet Source followers with still available parts to still have J-Fet inputs.

Of course, if you do all of this, it's no longer a Dynahi, but it should be excellent anyway.

Thor

* Sturgeon's law (or Sturgeon's revelation) is an adage stating "ninety percent of everything is crap". It was coined by Theodore Sturgeon, an American science fiction author and critic, and was inspired by his observation that, while science fiction was often derided for its low quality by critics, most work in other fields was low-quality too, and so science fiction was thus no different.

 

[farits]

After 2 months, an order arrived from Aliexpress. As a result, there is no sound. Although the indicators on the Zen can are on. On the plug itself, the voltage is 5 volts.

 

[Thorsten Loesch]

After 2 months, an order arrived from Aliexpress. As a result, there is no sound. Although the indicators on the Zen can are on. On the plug itself, the voltage is 5 volts.

5.6V is too high. It may trigger the Zen CAN protection.

You say "indicators are on", does that mean steady? Blinking?

Do you have the correct input selected?

It still works with the stock powersupply?

Thor

 

[farits]

You say "indicators are on", does that mean steady? Blinking?

Do you have the correct input selected?

It still works with the stock powersupply?

With the power supply from Ifi, everything is fine. The device is working.
With a Chinese power supply, the indicators are constantly on, as in normal mode, but where is the sound(((
I have a sad experience, shopping with Aliexpress(((

 

[dpump]

If you remove the cover for the power supply, you may have a potentiometer inside that will allow you to adjust the output voltage. If so, connect your multimeter to the voltage output connector and adjust for 5V. Most power supplies I have used have the ability to adjust the output voltage. Sometimes you may see the voltage adjustment on the rear panel instead of on the inside on the pcb.