[studeb]

I am referring to the 100nF and the 220uF caps. C1, 2, 5, 6, 7 and 8 in 272A.
If they are there to block DC, do we need all of them?

My kit arrived last weekend, with some other items from family in Oz. It has the black poly caps.

i am trying to figure out what else could be done. How about a lower value for the pots? You could double the plate voltage with two 9V batteries, the L4949 is rated for 28V, so it would be fine. Is there a family member with a higher regulated Vout? Also the datasheet recommends C12 < 1uF. i know others have put OPA2134 for the opamp. Not much else to do, it looks pretty nice.

Quote:

Originally Posted by sennsay /img/forum/go_quote.gif Hi studeb, which ones are you talking about? All of the 10uF caps are decoupling caps and that makes six all told and sure, they could be bypassed. If you've seen the pics I've posted, all of the blue caps are Phillips/Vishay 10uF electrolytic caps. If you meant the polyester box coupling caps, there are four of them and bypassing isn't really worth the effort, as you can just not use them and pop in there some 0.1uF or 0.47uF polypropylene caps (as Mark Houston has done) quite cheaply instead. However, in my case, since we are using a DC source here, I will probably just leave the 10uF caps alone - they're already very good caps - and use some of the wonderful little NX series 0.1uF Black Gate bipolar caps to replace the 0.1uF polyester box caps for the coupling duties. These things are stunningly good in that position and I've used them to great effect in my best upgraded version of the KHA II h/phone amp and other projects. Are you thinking of building one of these little tackers?

 

[netsky3]

Good news
I've bought a new solder iron and soldered the battery pack, the amp works almost perfectly, nothing was mounted wrong.
The only thing that i don't understand is why I hear a little distortion when i turn up a little the volume.
I used my pc as source with with the volume output setted as normal listening without the amp in the middle. (is right?)
Where do you set the volume? Source or trimmer?
My worry is if the amp cannot drive good my AKG without distortion.

Why my amp sounds bad if i turn up the volume? (only few steps)

Quote:

Originally Posted by studeb /img/forum/go_quote.gif i am trying to figure out what else could be done. How about a lower value for the pots? You could double the plate voltage with two 9V batteries, the L4949 is rated for 28V, so it would be fine. Is there a family member with a higher regulated Vout? Also the datasheet recommends C12 < 1uF. i know others have put OPA2134 for the opamp. Not much else to do, it looks pretty nice.

The tubes are rated for only 18V

 

[studeb]

Quote:

Originally Posted by netsky3 /img/forum/go_quote.gif The tubes are rated for only 18V

Data sheet mentions 30V max plate voltage. 22.5V for class A on the plate and G2.
Where did 18V come from?

 

[Pricklely Peete]

Netsky what impedance are your AKG's ? This little guy only has so much juice on tap and driving AKG's with this little guy is maybe a bit much for the amp to handle. Do you have some low impedance high efficiency cans to try out with it ?

The other possibility is the source output V may be slightly overloading the tube amp's input section but it's tough to say for absolute sure without knowing the amp's particulars. Maybe Sennsay can PM me or email me the specs and I can acquaint myself better with what this amp can or cannot do.

AKG's for the most part are current hungry difficult headphones to drive properly, that is especially true for the 701/702 series (the old 340's) and any of the 600 ohm Monitor series. OTL Tube amps are great at voltage swing but not so great at current delivery whereas solid state and hybrid (tube driver, mosfet/transistor power stage) depending on power delivery ratings into various loads are much better at delivering current and V swing. I believe this amp is a hybrid design is it not ?

That being said there is only so much this little amp can generate in that regard from a single dual channel IC OPA and it's 9V supply. It might be worth getting another kit and going balanced drive, doubling power which in turn will control the drivers of the cans with more of grip (so to speak). Sennsay's 18V PSU idea is another option worth exploring if the topology can tweaked to such a degree.

Peete.

 

[sennsay]

Quote:

Originally Posted by netsky3 /img/forum/go_quote.gif Good news I've bought a new solder iron and soldered the battery pack, the amp works almost perfectly, nothing was mounted wrong. The only thing that i don't understand is why I hear a little distortion when i turn up a little the volume. I used my pc as source with with the volume output setted as normal listening without the amp in the middle. (is right?) Where do you set the volume? Source or trimmer? My worry is if the amp cannot drive good my AKG without distortion. Why my amp sounds bad if i turn up the volume? (only few steps) The tubes are rated for only 18V

Netsky3, these tubes sound horrible for the first 2-4 hours!! Distorted bass and all. Give 'em a few hours and more and there will be a noticeable difference. My volume is from the dock port of an iPod Classic, so it is set, I control volume with the trim pots until my case is completed. If you are using the output from a computer, just out of the headphone outlet - which certainly does not do justice to this amp - I would leave the computer vol at say 8/10 and use the trim pots on the amp for the control of volume thereafter. There is also the possibility that the PT2308 isn't giving all that the AKGs need, better opamps (like the AD825s and OPA627s that I use, to name just two) may be more suitable. I don't have AKGs to test that out. Also, two 9V batteries in series to make 18V with the use of better opamps may also do the trick for you.

 

[sennsay]

Hiya Peete, have you been to this link? There is a circuit diagram near the top of the page and a link at the bottom to a discussion group:
Raytheon JAN6418 Valve (Tube) Preamplifier / Headphone Amp Kit

You answered some of what I posted above - I didn't look first

- I'm working on the 'upgrade' board now, the BG caps arrived yesterday

Just the NX 0.1uF caps, I'm going to leave the Phillips/Vishay electrolytics alone for now, they've worked wonders in the Plinius IIc preamp and they're doing just fine here too. The Nichicon 220uF FA's also stay, they're brilliant.
The BB2134 opamp has been mentioned a number of times, but seems to be preferred when the circuit is set up as a preamp, when the gruntier opamps are not used in this case to drive a set of cans. I really want the upgrade board to be used as a preamp into a KHA amp, see how it goes. I may need a metal case though, as there are earthing problems direct from the FrankenDac as it is - sounds just wonderful, yet any hum drops heaps in level when I touch an input socket with a finger. It's silent using an iPod.
To my surprise, this little amp gives the most out-of-head experience with the Denons that nothing else has, it's like having permanent crossfeed on, like mini Senns but a bit softer. Very VERY unfatiguing! I'm looking forward to hearing what metal film resistors and a few BG caps will bring.

 

[sennsay]

Netsky3, I'm currently listening to the little amp direct from my Frankie DAC and I have to tell ya even simple upgrades are well worth the effort! And yes, you will need very little from the volume input to go very loud, I have one of my favourite live albums of all time on at the moment, Neil Diamond's Hot August Night, and the results are incredibly alive and IN the event. As much enjoyment as I've ever had from a seriously good turntable or two! The bass is surprisingly excellent, taut and dynamic and music just flows effortlessly. All the natural husk in Neil's voice, the wonderful sound of the crowd, and really good natural sounding hand claps. If you think $440 SlinkyLink interconnects is nuts with a project like this, you'd be missing out on what this thing is really capable of doing

Not that you need to go that far, but it makes me even more keen to hear it as a preamp in my main system and a big Yamaha power amp attached to the end of it.
Like Peete, I just love killer budget kit like this, sounds great stock and responds wonderfully to some upgrades.

Studeb, you might be right mate, a vol pot with less range may be a very useful thing here, say 250k ohms, heck, even 100k may be fine with something as easy to light up as the Denons I'm using. I may even try a 100k Alps pot here, especially for the amp, might not be so useful for a preamp set-up, but there's only one way to find out.

PS, first experiments show that indeed the 2134/2604 opamps work well in the preamp set-up, as do the OPA627's so far (in fact very good, with fine depth and focus).

 

[netsky3]

Quote:

Originally Posted by Pricklely Peete /img/forum/go_quote.gif Netsky what impedance are your AKG's ? This little guy only has so much juice on tap and driving AKG's with this little guy is maybe a bit much for the amp to handle. Do you have some low impedance high efficiency cans to try out with it ? The other possibility is the source output V may be slightly overloading the tube amp's input section but it's tough to say for absolute sure without knowing the amp's particulars. Maybe Sennsay can PM me or email me the specs and I can acquaint myself better with what this amp can or cannot do. AKG's for the most part are current hungry difficult headphones to drive properly, that is especially true for the 701/702 series (the old 340's) and any of the 600 ohm Monitor series. OTL Tube amps are great at voltage swing but not so great at current delivery whereas solid state and hybrid (tube driver, mosfet/transistor power stage) depending on power delivery ratings into various loads are much better at delivering current and V swing. I believe this amp is a hybrid design is it not ? That being said there is only so much this little amp can generate in that regard from a single dual channel IC OPA and it's 9V supply. It might be worth getting another kit and going balanced drive, doubling power which in turn will control the drivers of the cans with more of grip (so to speak). Sennsay's 18V PSU idea is another option worth exploring if the topology can tweaked to such a degree. Peete.

Hi Peete!
My Akg are 55Ohms(240mkII), i have also an old pair of sennehiser px100 and the shure SCL4 (my first test was with my iAudio and shure and i don't remember any kind of dirstotion even i hear it few seconds

)
I'll try again later...

Quote:

Originally Posted by sennsay /img/forum/go_quote.gif Netsky3, these tubes sound horrible for the first 2-4 hours!! Distorted bass and all. Give 'em a few hours and more and there will be a noticeable difference. My volume is from the dock port of an iPod Classic, so it is set, I control volume with the trim pots until my case is completed. If you are using the output from a computer, just out of the headphone outlet - which certainly does not do justice to this amp - I would leave the computer vol at say 8/10 and use the trim pots on the amp for the control of volume thereafter. There is also the possibility that the PT2308 isn't giving all that the AKGs need, better opamps (like the AD825s and OPA627s that I use, to name just two) may be more suitable. I don't have AKGs to test that out. Also, two 9V batteries in series to make 18V with the use of better opamps may also do the trick for you.

Really noticeable? Good!

Now the amp is turned on for a little burn in, i'll try again setting the volume of my soundcard near 8/10 and use the trim pots to set the volume.
By the way, i have here a pot (like THIS) of 100K, do you think is good for this amp? where should be mounted? instead of the trimpot??

Quote:

Originally Posted by sennsay /img/forum/go_quote.gif Netsky3, I'm currently listening to the little amp direct from my Frankie DAC and I have to tell ya even simple upgrades are well worth the effort! And yes, you will need very little from the volume input to go very loud, I have one of my favourite live albums of all time on at the moment, Neil Diamond's Hot August Night, and the results are incredibly alive and IN the event. As much enjoyment as I've ever had from a seriously good turntable or two! The bass is surprisingly excellent, taut and dynamic and music just flows effortlessly. All the natural husk in Neil's voice, the wonderful sound of the crowd, and really good natural sounding hand claps. If you think $440 SlinkyLink interconnects is nuts with a project like this, you'd be missing out on what this thing is really capable of doing Not that you need to go that far, but it makes me even more keen to hear it as a preamp in my main system and a big Yamaha power amp attached to the end of it. Like Peete, I just love killer budget kit like this, sounds great stock and responds wonderfully to some upgrades. Studeb, you might be right mate, a vol pot with less range may be a very useful thing here, say 250k ohms, heck, even 100k may be fine with something as easy to light up as the Denons I'm using. I may even try a 100k Alps pot here, especially for the amp, might not be so useful for a preamp set-up, but there's only one way to find out. PS, first experiments show that indeed the 2134/2604 opamps work well in the preamp set-up, as do the OPA627's so far (in fact very good, with fine depth and focus).

Wow, if this amp can drive good my akg i'll immediatly buy a good opamp (and understand wich can supply more current to the headphones, opa627 can be good?) and the caps advised by you

 

[studeb]

Quote:

Originally Posted by sennsay /img/forum/go_quote.gif Studeb, you might be right mate, a vol pot with less range may be a very useful thing here, say 250k ohms, heck, even 100k may be fine with something as easy to light up as the Denons I'm using. I may even try a 100k Alps pot here, especially for the amp, might not be so useful for a preamp set-up, but there's only one way to find out. PS, first experiments show that indeed the 2134/2604 opamps work well in the preamp set-up, as do the OPA627's so far (in fact very good, with fine depth and focus).

i had a look at the schematics, the 100nf input caps and the 1Mohm pots make a high pass filter, if you use a lower value pot then you will loose some of the bass response. 250k would give a -3db point of ~7Hz. As is it now it is 1Hz.
If you use 18V i think you will have to change R9, not sure to what value.
If you change the op amps, you are still looking at a 5V swing, unless you swap the L4949 for a higher voltage regulator. For power hungry cans that could be the way to go.
i have to organise some jacks and should have mine running this weekend.

 

[studeb]

Quote:

Originally Posted by netsky3 Now the amp is turned on for a little burn in, i'll try again setting the volume of my soundcard near 8/10 and use the trim pots to set the volume. By the way, i have here a pot (like THIS) of 100K, do you think is good for this amp? where should be mounted? instead of the trimpot??

Netsky, you can use that pot instead of the trim pots, BUT you will loose some low end response, and you will not be able to balance the channels. As the tubes are not identical this COULD be a problem, and that is likely why there are dual mono pots, not a stereo pot. Maybe you could compensate with the balance on your sound card?

 

[netsky3]

Quote:

Originally Posted by studeb /img/forum/go_quote.gif Netsky, you can use that pot instead of the trim pots, BUT you will loose some low end response, and you will not be able to balance the channels. As the tubes are not identical this COULD be a problem, and that is likely why there are dual mono pots, not a stereo pot. Maybe you could compensate with the balance on your sound card?

As I tought...so, if the damage are sure to the audio I don't mount the pot, tought is very very unconfortable adjust the volume inside the case of the amp and not in the front panel

I'd like if the amp can be indipendent from the source...

 

[netsky3]

Quote:

Originally Posted by studeb /img/forum/go_quote.gif Netsky, you can use that pot instead of the trim pots, BUT you will loose some low end response, and you will not be able to balance the channels. As the tubes are not identical this COULD be a problem, and that is likely why there are dual mono pots, not a stereo pot. Maybe you could compensate with the balance on your sound card?

As I tought...so, if the damage are sure to the audio I don't mount the pot, tought is very very unconfortable adjust the volume inside the case of the amp and not in the front panel

I'd like if the amp can be indipendent from the source...

Peete and Sennsay: i try the amp with the output of the soundcard setted around 8/10 and the trimmer of the gain near the max but the volume was too low and distorced

I think that 55 ohm are too much for my cans
Instead with my shure e4c the volume seems to be a little better
I'm afraid that the amp or the opamp can't drive my headphone

 

[studeb]

Quote:

Originally Posted by netsky3 /img/forum/go_quote.gif As I tought...so, if the damage are sure to the audio I don't mount the pot, tought is very very unconfortable adjust the volume inside the case of the amp and not in the front panel I'd like if the amp can be indipendent from the source... Peete and Sennsay: i try the amp with the output of the soundcard setted around 8/10 and the trimmer of the gain near the max but the volume was too low and distorced I think that 55 ohm are too much for my cans Instead with my shure e4c the volume seems to be a little better I'm afraid that the amp or the opamp can't drive my headphone

you can mount the pots on a panel and run wires from them to the board
what soundcard are you using?
try using a cd player to verify the amps performance

 

[netsky3]

Good news mates,
I've resoldered the tubes and the apamp, changed the 2 caps with a pair of the same value (C7-8 220uF) that i have here and the sound seems to be better, much better.

The volume is good but a little of more power would be helpful in some cases.
Now i do also a little circuit to mount between the psu and the amp, i suppose.
I'm using a psu at 12V - 1A, if i'll use something at 18 for example, can be useful to the output volume or is indipendent from the voltage??
Another question: if I mount caps with a bigger capacity (in C7-8 or 1-2-3-4) change something in the sound?

Quote:

Originally Posted by studeb /img/forum/go_quote.gif you can mount the pots on a panel and run wires from them to the board what soundcard are you using? try using a cd player to verify the amps performance

No, at the moment i can't use a cd player, i have here only the pc and and my cowon i7. I'll back at home in the next two weeks

 

[netsky3]

Peete, now that my amps sounds very well I can think to the upgrades.

First of all the opamp. There's differences about the output power or are similar?? If they're similar I choose a OPA627 that sounds good and it's not so expensive (about 10$ shipeed in Italy).

Caps: Apart C7-8 that they capacity can't be changed (right?) I've looked again your pics and I've noticed that your caps (C3-4 11 12 13) are very different from the mines. They can improve the sound?

To have a better output power an expert in electronic told me that C7-8 could be changed but cutting off the frequencies under 40HZ.
I don't know if do this experiment...