Which audio buffer design is the best for Gainclone?

[Franz G]

Here my driver board:

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

This is the second one, so both monoblocks are playing now with opamp drivers.

I added by scratch the part for the printtrafo and used some 1uF caps out of the hobby box, thats why it looks a little bit ugly...

And I forgot the cap in parallel to the feedback resistor: two twisted, isolated 1 inch long wires.

Franz

P.S.
One channel uses now fast schottky diodes, the other one just this 1N4007. It seems to be no difference in the sound.

 

[gengcard]

Hi Franz,

some german engineers told me, 7815/7915 are not the best choice

Thanks for the suggestion. I will tried by another device.

Hi Carlos,
Yesterday night, I changed from OPA627 to OPA228 already. After I listened its sound, I still agree with you OPA627 better. OPA627 has very good bass.

I'm interesting in the biasing opamp into class A. Please suggest me about this.

 

[Peter Daniel]

I also decided to keep my OPA627 in ML preamp. I didn't like AP grade but BP sounds pretty good now.

 

[gengcard]

I didn't like AP grade but BP sounds pretty good

I have only AP grade, however, I think its sound is also very good. I will try BP grade later if I can get its sample.

 

[matjans]

carlos recommends using a single resistor between v- and output, although it is theoretically an inferior option. other people prefer a transistor based (optimal, steady) current delivery system.

I've tried both biasing tricks on a pimeta headphone amp (based on ad8610's/buf634's). I personally couldn;t hear the difference between the resistor and transistor biasing methods. For me, biasing the opamp into class a helped a lot in the high-mid section. It cleared up the highs and mids and got more 'open'. In the bass section i couldn;t hear any difference.

I use(d) the pimeta headamp pcb as a preamp line driver. It sucks, totally messed up the basses. Way to boomy to my taste. But then again, the pimeta is a headphone amp and it has some serious grounding issues (where's the stasr ground?).

Next stop: ad8610 class a on veroboard close to my (amp to be regulated opa549.

 

[carlosfm]

I'm interesting in the biasing opamp into class A. Please suggest me about this.

Here it is:
http://www.diyaudio.com/forums/showthread.php?s=&threadid=38984

I use(d) the pimeta headamp pcb as a preamp line driver. It sucks, totally messed up the basses. Way to boomy to my taste. But then again, the pimeta is a headphone amp and it has some serious grounding issues (where's the stasr ground?).

It's the lowsy layout and PSU bypassing.
Yes, that's exactly what it does to these op-amps.

 

[gengcard]

I have checked the price of OPA627 between AP grade and BP grade, BP grade is expensive than AP grade. Does the price make sound different?

I've tried both biasing tricks on a pimeta headphone amp (based on ad8610's/buf634's). I personally couldn;t hear the difference between the resistor and transistor biasing methods. For me, biasing the opamp into class a helped a lot in the high-mid section. It cleared up the highs and mids and got more 'open'. In the bass section i couldn;t hear any difference.

Is the conclusion the biasing by resistor is good than transistor if we mention to their prices. For me, I will try to use cheap devices but have a good quality to make a very good amp.

 

[jean-paul]

Franz, do you have any comment on my remarks in post #27 ? I see you removed the 47k feedback resistor as suggested. Like Carlos I also recommend the use of a regulated PSU for the OPA as it likes low impedance supplies. I won't start on grid stoppers, I promise

When I use OPA627/637 in most cases I use a 10 nF 63 V MKT cap soldered from + to - directly on the pins ( the cap glued with a tiny drop of glue to the chips body ). When I first did that I could not believe that this made such a difference but it did. OPA627/637 is very sensitive to decoupling, contrary to other types the use of relatively large low ESR electrolytic decoupling caps from + to ground and - to ground is recommended. As known don't use the low ESR caps right after the regs if you use those but use them as close to the chips pins as possible.

BTW the differences between AP and BP should not be exxagerated. BP is the tight specs version and most AP examples sound more or less the same as BP. Only the ones that really are close to their max. tolerance sound somewhat different. This is negligable compared to the extra cost of BP in most cases !. They are both from one and the same production line.

 

[Franz G]

Jean-Paul

I am sorry, but actually I am to drunk to answer your question seriously.

But I printed out your question and will answer tomorrow. In the meantime you may have a look at my latest amp enclosure (shellack polish on MDF not finished yet) for my pure (OPA627 buffered, inverted and regulated) LM3875T chipamp.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

Goodnight and Sawadee (krap) to Bangkok
Franz

 

[Upupa Epops]

MDF heatsink is the best, isn't truth Frank ?

 

[Franz G]

MDF heatsink is the best, isn't truth Frank

My name is Franz, this I still know, hips...

What sink?

Franz

 

[Upupa Epops]

I'm sorry Franz . But I only mean, that MDF is not quite ideal material for chassis, inside which is " temperature supply " . Remember, that by 10 ° C lower temperature rise reliability 100 % .

 

[Nuuk]

I'm sorry Franz . But I only mean, that MDF is not quite ideal material for chassis, inside which is " temperature supply " . Remember, that by 10 ° C lower temperature rise reliability 100 %

I take the point but it is only a general rule. If you design your heatsink and airflow properly, the case materials should not be a very large factor in the temperature of the chips.

 

[Upupa Epops]

To Nuuk : How I see, you like wood - me too, it smell good ( in the my next life I will be joiner ), but air conditions are in this case rather problematic .

 

[Nuuk]

Upopa - go all the way - in your next life be a tree!

If you look at my wooden cases, you will see that in each case I ahve taken care to make sure that the chip is well-cooled. None of my amps have got hot and most of them run at little more than body temperature.

I'm sure that Franz is expereinced enough to realise that there may be a heat problem with a wooden case and take precautions (at least when he is sober )

 

[Franz G]

Good Morning!

No, heat is no problem, as there is a big Fischer Heathsink inside. The amp worked as breadboard amp before and never got warm.

And I provide some airflow inside the box: there are a bunch of holes on the under side.

Back to the tube outputstage version:

Some remarks: you could use a 100k or 220k resistor from the non-inverting input to ground ( imagine what will happen if the wiper has a bad day ).

This is quite a good idea...

The use of an RC filter at the input is preferred as OPA627 has quite a high bandwidth and it likes a series resistor too at its input. Make that filters -3 dB point around 100 kHz.

I am not a friend of filters, but I think I will try it and compare. It seems really contraproductive to send RF-frequencies to the output tranny.

Probably the use of an input cap ( when it is unknown whether the source is DC free or not ) would be a wise choice but when it can be avoided you should avoid it of course. Did you listen to it like it is now without the 47k overall feedback resistor ?

DC should not be a problem, as the output transformer blocks DC out to the speakers. It will just get the tube out of the optimal working point, but I should remark this in the worst case.

As you have already seen, I removed the overall feedback and the tube works now with current feedback (no cathode bypass cap). I worked out, that I prefer the triode mode in this configuration.

BTW I don't understand your remark on the asymmetrical output stage of the LM3xxx series when I think of Single Ended tube amps ( or even more: balanced tube amps ) !?!?!

Sorry, I think the correct word is "pseudo complementary" output stage.

Generally, I prefer Single Ends in Class A. I sold all my PP-Amps...

But this is just a matter of taste, imho.

Regards
Franz

BTW: I fact I don't use originally EL84, I use russian equivalents 6pi14pi-EB, a very good military version. Cheap and good source: seller KWTUBES at ebay. Sends very quick and reliable out of Lituania. Source for very good caps and other things...

 

[Upupa Epops]

Be a tree - splendid idea, I will try it .

 

[Franz G]

Inside the MDF box, the "regulated chipversion" looks like this:

An externally hosted image should be here but it was not working when we last tested it.

Don't worry: the blue cables are thermal resistant silicone cables and the yellow/green does not touch the heathsink.

Franz

 

[Upupa Epops]

Franz, if I may, only little note : common ground wire to the speakers terminals give you crosstalks between channels. Green / yellow conductor MUST be used only as PE, norms are norms !

 

[Franz G]

You are absolutely correct, I should not use yellow/green conductor (I am not a profi, just building amps for my satisfaction...).

Another bad thing: the PE hangs in the air, as there is no chassis to connect it. Maybe I better use a two wire powercable. Then it will be obvious, that there is no PE connection.

I will separate the output conductors. Thanks!

Franz