PPA v2 construction discussion

[FallenAngel]

Not so good is the $20 MasterCRaft 052-0060-2 from Canadian Tire. It's a pretty basic meter, nothing fancy, but works well. The other is a cheap little one that has HFE testing so I don't have to bother making a circuit.

The funky this was that both show the same value at about 135mA current draw when there should only be 100mA.

 

[pobrecito]

Hi, Does anyone know what is the lowest real world gain I could use with OPA637s in the left & right channels. I know that the data sheet states >=5. I'ld like to reduce the gain in order to allow for better use of the volume control.

 

[MdRex]

More questions:

I read that the total capacitance of the power rail caps (C1) is kept at the range of 1000-2000 uF but I've seen many pictures of people adding 8 high capacity caps (say around 1000uF ea). Wouldn't that mean the total capacitance is around 8000uF and will there be any negative effects if the capacitance is >2000uF?

Another thing, is it necessary to use caps from the same line in the power cap, I ask this because I have 6 Panasonic FC 220uF and 2 Nichicon Muse 1000uF cap and I was wondering if I could use them together on the power rail caps (C1)?

If not, should I then use the 6 FC caps or the 2 Muse caps?

Thanks!

 

[Epicurean]

Quote:

Originally Posted by pobrecito /img/forum/go_quote.gif Hi, Does anyone know what is the lowest real world gain I could use with OPA637s in the left & right channels. I know that the data sheet states >=5. I'ld like to reduce the gain in order to allow for better use of the volume control.

From what I've heard a gain of 8 is recommended. I tried a gain of 6 but it would turn unstable at random while listening, so I adjusted it to 10 and it works perfectly.

IMHO, after extensive listening with OPA627,OPA637 and AD8610, I find the AD8610 to be the clear winner with no apparent faults, especially with senns. The OPAs do add some warmth, but the bass is off, sounds muddy and bloated in comparison to the AD8610. And yes I tried this with about a dozen different sources. This was also the case with DT990s '05 and every cheapo headphone I have laying around.

 

[tangent]

Quote:

Originally Posted by pobrecito /img/forum/go_quote.gif Does anyone know what is the lowest real world gain I could use with OPA637s in the left & right channels.

Don't push it. The 637 is notoriously difficult to make stable in the PPA in gains over 5 already. If you want to keep that same basic sound with a lower gain, switch to the 627...that's what it's for.

Quote:

Originally Posted by MdRex I've seen many pictures of people adding 8 high capacity caps (say around 1000uF ea). Wouldn't that mean the total capacitance is around 8000uF and will there be any negative effects if the capacitance is >2000uF?

As supply capacitance goes up, the power-up and -down thumps are larger, and it can overstress some power supplies. I have yet to see anyone A/B test a 2000uF PPA vs. a 8000uF one, so I still believe the audible advantage is nil. 2000uF is already an uncommonly large amount of capacitance for a headphone amp.

You see PPAs with thousands of mics of rail capacitance in them only because it can be done, not because it gives some real advantage. We put 9 rail cap positions into the amp to account for boutique caps, which are often only 220uF in in the physical size required in the PPA. If we were designing for 1000uF caps, we'd have put 2 cap positions in there instead, and used the space for something else. We had ideas for lots more features we could have put in, if the space was there.

Quote:

I have 6 Panasonic FC 220uF and 2 Nichicon Muse 1000uF cap and I was wondering if I could use them together on the power rail caps (C1)?

It'll be fine.

But, why don't you save the FCs for another project? Just because the holes are there, doesn't mean you must fill them.

 

[ppl]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Just had a chance to play with this amp some more and here are the observations. I took all the opamps out, adjusted the bias to as low as it would go on each channel. Ground = 20mV = 9.1mA Right 16mV = 7.3mA Left 12mV = 5.5mA Total should be 22mA, I got a current draw of about 40 When I have all opamps installed and adjust the buffer bias from lowest to 25mA each, the DC offset changes from an initial -2.0mV / 0.8mV to is -2.3mV / 0.8mV on the other. Definitely funky stuff

IMHO your missing Ma is from mismatched Op-Amp Rail Isolation JFET's. If the Idss of each fet is different then the difference is expressed as excess current drain. If you have a set of op amp rail isolation FET's with mismatched Idss then any imbalance will simply Pull upon the output of the TLE-2426's causing a current to flow.

The Bottom like is is you are building a PPA for portable use you want the lowest battery drain as possible, it is now required that the rail isolation fets be matched for Idss,It's not just a nice for battery operation. Having missed matched JFET's can added 10-50 Ma additional current drain. Oh Ya BTW matching transistors in the output stage while not required will reduce open loop Distortion up to 10 times.

As with any DIY project the care in assembly quality of the solder joints and how cleen the PCB is all matter allot.

 

[FallenAngel]

Quote:

Originally Posted by ppl /img/forum/go_quote.gif IMHO your missing Ma is from mismatched Op-Amp Rail Isolation JFET's. If the Idss of each fet is different then the difference is expressed as excess current drain. If you have a set of op amp rail isolation FET's with mismatched Idss then any imbalance will simply Pull upon the output of the TLE-2426's causing a current to flow. The Bottom like is is you are building a PPA for portable use you want the lowest battery drain as possible, it is now required that the rail isolation fets be matched for Idss,It's not just a nice for battery operation. Having missed matched JFET's can added 10-50 Ma additional current drain. Oh Ya BTW matching transistors in the output stage while not required will reduce open loop Distortion up to 10 times. As with any DIY project the care in assembly quality of the solder joints and how cleen the PCB is all matter allot.

Well that would definitely explain some things since I didn't match the Idss of the FETs.

I did however match the output transistors for HFE so I'm hoping distortion (although I can't hear any) is kept at a minimum.

Unfortunately though, that might not be the problem since I just removed all the rail isolation JFETs (Q3), matched new ones as closely as I could for IDSS (got the pairs within 1mA of each other, my PN4392's had quite a range, some at 28mA, others close to 50mA). The pairs I installed in this amp were 32.2/31.8, 42.2/41.9 and 46.5/46.3, I'm assuming it's only the pairs I need to match, not all together, right? After installing the new matched ones though, I still get 130mA (and slowly climbing at power-up) current draw when the buffers are set at 25mA.

 

[ppl]

Quote:

Originally Posted by FallenAngel /img/forum/go_quote.gif Well that would definitely explain some things since I didn't match the Idss of the FETs. I did however match the output transistors for HFE so I'm hoping distortion (although I can't hear any) is kept at a minimum. Unfortunately though, that might not be the problem since I just removed all the rail isolation JFETs (Q3), matched new ones as closely as I could for IDSS (got the pairs within 1mA of each other, my PN4392's had quite a range, some at 28mA, others close to 50mA). The pairs I installed in this amp were 32.2/31.8, 42.2/41.9 and 46.5/46.3, I'm assuming it's only the pairs I need to match, not all together, right? After installing the new matched ones though, I still get 130mA (and slowly climbing at power-up) current draw when the buffers are set at 25mA.

Remove ether the TLE2426 or the Rail JFET's and then remeasure the current drain what you should have done to eliminate this phantom current is see if any current is drawn when only the rail isolaten JFET's and the TLE2426 are installed no op amps no buffer parts then you would know where your phantom current is com9ong from and in your case its one of two issues (1) the JFET mismatch as previously described or you have mile oscillation in the output stage however this would also show up as an increase in voltage across your emitter resistors. In any event all anyone hear can do is suggest possible reasons for the malfunction it is up to you to investigate these.

another thing to consider is the current the drivers and the Bias current generator of the output stage consume and this is about 1/4 the current you measure across the emiter resistors on the output transistors. measure the voltage drop across the 10 ohm emiter resistors for the driver transistors add that to your total.

 

[MdRex]

I've populated my PPAv2 board and after installing the OPAMP (AD744) I've got extremely high DC off set (several hundreds millivolt), the sound range from having the right channel only, no sound, to severe distortion. It's frustrating.

 

[MdRex]

I wonder if the problem is my output buffer stage. The transistors are not getting warm. I've only felt warmness in the MJE243/253

I only hear severe distortion when I turn the volume up in the right channel, the left channel can barely be heard. I am using R4 to be 5.6K to lower gain and the rest of the parts are the same as what tangent has on his site.

Can anyone help me please?

 

[villekille]

The current consumption seems like a common problem for the PPA. I have a PPA v2 with AD843 opamps and NO class A biasing, standard parts, 15V power supply and the current consuption is 163mA.

The output bias is set to 20mA if I have understood it right. (4.7 ohm resistors at R24/R34 and the voltage across them is 100 mV)

I don't know where all that current goes but the amp sounds fine

 

[villekille]

I removed all opamps, and Q3s and the current draw was about 130mA. The buffer bias was set at 20mA.

Then I removed Q4s, one by one, and they all had about an equal effect. The current draw went down to 1.4 mA after removing all of the Q4s.

So each buffer consumes about 45 mA when the bias is set at 20mA..(voltage across a 4.7 ohm resistor is 100mV).
There aren't any differences between the channels. That's more than 2 times the bias current!
My Q4s are "2N4392".

 

[MdRex]

Is there any fool proof way to test if the buffer section is faulty? Someone offer their help please

 

[squid]

Hello everyone. I was hpoing someone could give me a clue whats happening with my ppav2. I have weird output bias currents: on left channel it wont go lower than 100 mA, on ground it wont go higher than 19 mA and on right channel its stuck on 6 mA. R12's have almost no effect on these currents. R32 and R33 have 10 volts across them, does not seem right either. Weird stuff start with buffer section, all voltages are fine before it.

All using 24 Volts supply. Would greatly appreciate any comments.

 

[Clutz]

Double check that you have the correct value resistors in each of the positions. Also, you might want to consider taking some pictures of the top and bottom of your board (high quality photos in macro mode).