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QRV-08 Building and test

Started by galouche, June 14, 2007, 06:44:29 PM

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yes, but its -possible in other designs to have the amplifier operating in class A even when its not at full volume and the write-up says the amp is already class A, but (I understand that this is not always the case depending on the load, and also you forget, this amp will ALWAYS be at full volume, i'm using digital attenuation and there is no pot to speak of. and this is also why low low noise is very important to me thus the thought of finding a lower noise reference, because full volume into 30R headphones could produce noise, I guess maybe i'll cross that bridge when /if I come to it

I was only looking at better references because you mention in your other material (and this is confirmed by my and many others) that LM329 is a much higher quality reference  and not a small amount better, but then you say SMD is better. I will probably end up using TL431 anyway, but you stil havent responded as to whether the fact something is SMD makes it THAT much better that it all of a sudden is better than a considerably superior device.  if the answer is yes, I will probably use 431, if not I see no reason not to use 329 and keep the leads as short as possible.

I know the design is hard to tweak (and thats a good thing), its an incredibly developed design based on some really incredible lineage, which is why I chose it. changing the reference is not really much of a tweak, but if you dont want to make a recommendation then OK.

the only other changes I was looking to make (aside from dual mono) is omitting the input cap if possible, because no cap is better than a good cap, I have installed the trimpots. and maybe further down the track using RFB03 to feed the DC instead of the onboard rectifiers but I wouldnt expect you to support that one, other than perhaps the build of the RFB03 itself if I had any queries


Class A:
This has nothing to do with volume. It's about the load and how heavy it is. If the class A current is 10 mA, this will mean that the amp is starting to work i class B for output currents (peak values) over 10 mA.

Low noise:
This has nothing to do with the volume. It's determined by the input transistors and the source impedance. A high ohmish pot at - 6dB attenuation will create extra noise. The amp itself has a bit lower PSSR than a normal opamp so it's more important with a good power supply. Noise can seek it's way into the amp but the used reference will have a very low impact on this because of the lowpass filter after the reference. Using a better reference than the LM431 is only required iof you want to feel better. Since you not are going to measure anything many of the parameters of the reference are not important. A plain zener will do but it's nice to use a precision part but not necessary.

Full volume into you 30 Ohms earphones will probably blow them.....

Sure you could use RFB03 to feed the board but don't forget that short wires and compactness is a virtue when you are dealing with wideband designs. Your long wires may pick up noise instead.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff


I will not have the volume turned all the way up, it will be attenuated at the digital level in the DAC registers, but the amp does not have any attenuator at all as is the case pretty much any time you use a digital attenuator, so the amp is effectively at full volume, so it is as important for the amp to be low noise as it is the source, as the source effectively takes the place of the pot, so yeah I see what you mean as the situation is the same, but the issue is just moved back a place. lets just hope nobody ever unplugs the amp while i'm listening, the resulting pop would be pretty loud!, lucky I dont have to worry about the connector shorting when its plugged in or removed because its not a phono, all the same I will never be doing it with them in my ears.

oh and it actually will be being measured, this is being used as part of the development phase for a dac, testing different output stages. but that is unimportant to me

I wont deny that it is in part to make myself feel better, as with a number of things I and I dont doubt, you do at times (though i'm thinking not that much after this convo), but its OK, if you dont know of a better part then I will search one out myself after the initial build, I will just go ahead and use the hole mounted 431 or 329 now and see, then when I get the itch i'll change the ref to something else (I have to order SMD ref). I dont believe there isnt a better part as 431 is really a pretty plain vanilla part, maybe perfect for the job, maybe already over what the job needs, but for some reason you avoid answering a pretty straight forward question

is it more important to use an SMD part because its SMD than using a higher quality hole mounted part?

why does being hole mounted make a difference if using a better part does not?

lets put aside for a moment your belief that it wont make much/any difference to what I actually hear and give me a theoretical answer...please!! I have asked 3 times now.

I have hole mounted 329 and 431 right here, right now and it is the only thing stopping this amp from being finished, all the small signal T's, r's and C's are there, the larger transistors are there, the rectifiers are in place, the opamps are in place (except for 2 of the 8 AD825), all that is left of the small parts is the references and then I can go ahead and mount the rest of the large parts, once the right transformers arrive i'll be finished, the only reason I was asking about a better SMD part, is because if i'm going to make a new order for no oter reason but to get a reference and a few opamps, it better be a good part.

whether it makes much difference at this point means very little to me, feeling I have done the best I can do to get the best result in this build is very important to me, as is evidenced with the budget and the fact I have soldered probably 500-600 joints doing most of the work for just the headamp section of this dac, with at least the same again for the dac and reg section.

whether you like it or not PA, you are in a market which, funnily enough, has quite a few audiophiles and even though i'm not totally there, I do have some tendencies that way, so if you sell these extreme audio products of yours to extreme audio technology enthusiasts, you will just have to live with that, I have seen you make comments which make it sound like audiophiles are silly, I dont know if this is intentional or not.....

the amp is always wide open, no pot = no attenuation just the fact i'm using such sensitive headphones is enough to want it to be as low noise as is possible, not only are they low impedance, they are probably one of the most revealing and detailed/fast headphones on the planet. the common mode will take care of most of it too though I guess.

QuoteSure you could use RFB03 to feed the board but don't forget that short wires and compactness is a virtue when you are dealing with wideband designs. Your long wires may pick up noise instead.

cool, yeah agreed, the idea is to make my own PCB to integrate the power lines and sensing for the regs, with a large ground plane for the rest and ideally the amp would connect by pin headers or at worst very short wires.
will let you know how it goes, should flick the switch early next week on the amps


Hi there, after many hundreds of solder joints I have both PCBs stuffed, but before I power one of them up to test, I would like to know if I should be expecting the LEDs to light up or they are oriented so they dont? I have not seen any pictures in here that have them glowing at all, so I wonder what to expect??



The LED's will glow quite much. Before you solder, make sure where the cathode is. Use a DVM in diode test mode. You will see some light from the LED.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff


lucky you reminded me to test myself rather than relying on the datasheet. the datasheet reverses the orientation for the diagram for polarity. there are 2 diagrams next to it that are up one way and the 3rd one that shows polarity is flipped upside-down. so desoldered them tested and yes they were wrong (lucky I asked, I have had LEDs with wrong polarity do very nasty things before) so installed the correct way, flipped the switch and 4 pretty red LEDs lit up ;), then a few seconds later the left one went out :(. reflowed all the joints in the left regulator switched it on again and its firing on all cylinders!!!

I have not connected any headphones yet, I want to check the other build first, most likely i flipped the LEDs on that too. then i'll go through the test procedure; expect to hear from me in the next couple of days.

anything specific I must keep in mind when adjusting the pot for the servo? unfortunately I couldnt get any of the model with the pot on the top, I could only get side mounted, so it will be a bit awkward to adjust it



and another, how do I place the pic directly in the post on this forum??
BTW these pics were taken before I remembered to short R/E56,57 and remove the compensation caps


Uploaded pictures are presented as you'll see but if you want in the test you must host pictures yourself.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff


  I don't find the set of 10,2 mA in the last stage output as mentionned, in simulation I find 1,8 mA so I've mesured the r20/r22 potential difference and it give 1,8 mA too

Is there something I don't understand ?


That is correct and also according to my measurements. I will change my text.
/Per-Anders Sjöström, owner of this forum

Homepage with my DIY hifi stuff


thanks for your answer as I'm a newbie I study this very good headphone amp and in wich class it works that's why I've found this.