Hi Carl
I've tried a number of different buffers with the PGA devices.
OPA134, OPA627, AD797, LME49710.
Let me clear something up, the PGAs when used properly on a well designed PCB are very good! but still they are missing something... i can't quite describe it.
Regarding the buffers, OPA627 appeared to have the best performance, very neutral, the LME had the most detail, excellent clarity and speed, the OPA134 was missing a little bit of both and the AD i can't remember, tried it very briefly.
Cheers,
I've tried a number of different buffers with the PGA devices.
OPA134, OPA627, AD797, LME49710.
Let me clear something up, the PGAs when used properly on a well designed PCB are very good! but still they are missing something... i can't quite describe it.
Regarding the buffers, OPA627 appeared to have the best performance, very neutral, the LME had the most detail, excellent clarity and speed, the OPA134 was missing a little bit of both and the AD i can't remember, tried it very briefly.
Cheers,
Altho I haven't actually built this circuit, I have built many that are similar to the one that I have attached. It is from a thread here on the forum. My apologies for not remembering it's author. Perhaps someone here can offer that information. However I have to believe if it were slightly adjusted (ie: lose the input pot) it would make a killer preamp when paired with a PGA2320.
I think it Juma, here's the link;
http://www.diyaudio.com/forums/showthread.php?postid=1690625#post1690625
But this is not a buffer. It would make more sense to simply use a diamond buffer ahead of the PGA. Or a good opamp buffer. There is a not so great opamp inside the PGA so how much more damage can an external one do? Of course one may wonder if all these complications make it still worth it to use a PGA. There are lots of electronic pots which will work as good or better with the help of a buffer/gain stage.
I simply disagree,
In my experience the op amp inside the PGA2320 part is perfectly acceptable. I can only speak to my experience with the PGA2320 as I have little experience with the other parts in the family. For me the internal op amp simply has no sound.
As to the circuit not being a buffer, the gain can be easily adjusted to unity. And in my experience (please take this with the appropriate grain of salt as it is my subjective opinion), these simple FET/ BJT combo circuits are easy to build and sound great.
In my experience the op amp inside the PGA2320 part is perfectly acceptable. I can only speak to my experience with the PGA2320 as I have little experience with the other parts in the family. For me the internal op amp simply has no sound.
As to the circuit not being a buffer, the gain can be easily adjusted to unity. And in my experience (please take this with the appropriate grain of salt as it is my subjective opinion), these simple FET/ BJT combo circuits are easy to build and sound great.
Carl_Huff said:
That's fine and the thread starter disagrees with you from the first post
I have a box with practically every audio opamp from the last twenty years. While several are very acceptable, all of them have easily identified sound in comparison to a bypass. And i only feed them the best power money can buy 
It appears the opamp inside the PGA is comparable to an OPA134 but certainly not to the best available.Actually as far as i know it is the same design as the OPA134 modified to have a variable gain. That's why you don't need another buffer on the output (up to some load).
I did a lot of experiments with the PGA2320 trying to improve it. Buffers on the input and output, filters, only on the input, with no buffers (with controlled source) and many more.
What i found was the perfomance improved significantly when i did an actual PCB, because most of the testing was done on veroboard. On the fabricated one the improvement was massive and i'm pretty good with veroboard. I suspect that this had to do with the partial planes i used, for the digital and the analog ground. Carefull when i say plane.. i don't mean exactly plane for the analog, more like a directed track/plane.
I tried to get the best out of it on that PCB and realised that the PGA as i said before is good, excellent in fact for many applications... if i was building for example a mixing console that would be my number one choice, or an integrated amp for the hifi consumer market... but it's nothing special, it's not something i would use on an amp/preamp that i want to sit down and enjoy listening music from it.
The shock came to me while i was tesing my BPA in an audiophile studio, connected to a pre amp (i can't remember which one) of the >15k£ class, and the acapella Lacampanella speakers, and some other equipment even higher class than this. I couldn't believe that i was still listening to my BPA... When i did the comparison to it with my pre amp based on the PGA the amp was literally dead.. no life whatsoever.
And comming to realise that now, that actually my description of the PGA sound... it has no life!
One last comment on that is that it hugely depends on what kind of music you want to listen to... If you're looking for club music.. trance, beats etc... the PGA is perfect... but once you get to something with pianno, drums, sax, and most of all voice.. simply the PGA didn't see it comming!
My 2 Cs.
On another subject...
I've been reading today about the different resistors and found out about these tantalum ones... honestly never heard of them before. Not only that never heard of them but i can't find any documentation on them... i mean real documentation from manufacturers, something that explains the technology.
I've used in the past thin film SMT resistors on designs that required SNR of 180db and circuits that required to pick up signals down to nV, with excellent performance...
Have you guys heard such a huge different in the actual result between thin film and tantalum? Because honestly if i'm going to spend 3£ per resistor i'd like to know why
I've been reading today about the different resistors and found out about these tantalum ones... honestly never heard of them before. Not only that never heard of them but i can't find any documentation on them... i mean real documentation from manufacturers, something that explains the technology.
I've used in the past thin film SMT resistors on designs that required SNR of 180db and circuits that required to pick up signals down to nV, with excellent performance...
Have you guys heard such a huge different in the actual result between thin film and tantalum? Because honestly if i'm going to spend 3£ per resistor i'd like to know why
Carl,
I didn't say it that my "BPA" is THE amp... quite the opposite, i'm actually looking forward to a new design with many many improvements. I was surprised to be honest with it's performance, overall it has an excellent sound, but has weaknesses.
I'm preparing another post dedicated to this BPA and the people who helped me (from this forum) two years ago when i designed it.
Unfortunatelly the only chipamps i've listened to are the ones i've built so i couldn't tell you if they are better or worse that the ones other people have built.
One thing that i've done diferently than most people is the power supply. I built my own regulated PSU. I know it's a controversial subject, but i do think that it has a big impact. I'll post all the details soon and i'm actually very interested to see what people think.
I didn't say it that my "BPA" is THE amp... quite the opposite, i'm actually looking forward to a new design with many many improvements. I was surprised to be honest with it's performance, overall it has an excellent sound, but has weaknesses.
I'm preparing another post dedicated to this BPA and the people who helped me (from this forum) two years ago when i designed it.
Unfortunatelly the only chipamps i've listened to are the ones i've built so i couldn't tell you if they are better or worse that the ones other people have built.
One thing that i've done diferently than most people is the power supply. I built my own regulated PSU. I know it's a controversial subject, but i do think that it has a big impact. I'll post all the details soon and i'm actually very interested to see what people think.
This is the chipamp forum... BUT
Ive had excellent results from my John Broskie Aikido boards, theyve been going strong for nearly 2 years now with some 6SN7's.
I combined with a dual mono PSU with 12H and 1000uF per channel, EI HT traffo, a tube rectifier, and dedicated DC regulated heaters, also a stepped attenuator volume control.
Cant go far wrong if you need the gain, and low impedance drive!
When the Aikido was hot news i believe most people gave it rave reviews, dont know if its been beaten yet though?
Ive had excellent results from my John Broskie Aikido boards, theyve been going strong for nearly 2 years now with some 6SN7's.
I combined with a dual mono PSU with 12H and 1000uF per channel, EI HT traffo, a tube rectifier, and dedicated DC regulated heaters, also a stepped attenuator volume control.
Cant go far wrong if you need the gain, and low impedance drive!
When the Aikido was hot news i believe most people gave it rave reviews, dont know if its been beaten yet though?
otherside said:
A pretty good description of what is wrong with the PGA subjectively. Having tried some of the electronic pots i am beginning to think that the opamp is not the main culprit, it's the fake resistors.
I have been using tantalums since 1985, initially Shinkoh and later on AN. These days they appear to have gone mainstream although subjective appraisal of the SMD types varies. Disturbingly, most of the older ones i have have drifted significantly in value, especially if used to pass any meaningful current. They have a very attractive sound character although i would not call them neutral.
OK so, lets forget the PGA for a while now moving to other things.
I've spent quite a bit of time reading and reading threads related to subject in this forum and others.
The Lightspeed or in general preamps with LDRs seem to be in common agreement by everyone that perform better than anything... it could well be true, honestly haven't tried one yet, but i'm for sure going to try them out! Just ordered a few of them.
I've been following the Lightspeed thread and i've seen that no one (at least from the forum) has taken the difficult path to actualy make the best of them integrating some digital and software control. So as my expertise (and my job) is with this kind of things, i think i'm gonna give it a try, a few DACs and ADCs, a nice little micro, calibration routines etc. No need to match them and absolute control. So that's what i started working on.
But before i got to that conclusion i was looking how best to implement a stepped attenuator. I've read whole trains of documentation, forum threads etc. and my conclusion is that possibly the best way is using iCMOS switches. So i started working on that design as well, did the prelim schematic, started looking for the components.. and that's where i got stuck!. The cost to build a 64 stepped attenuator using good quality components and ICs is more than 200£ only the components to start with plus the cost of the PCB. The other problem i realised is that the signal path length is going to be long, so some tricks are required on the layout to actually keep it on the shortest possible length, after all 64 +1 resistors are a lot! The plan was to use 0805 thin film 15ppm 0.1% resistors, but i think to get it on a proper size and make the signal path acceptable will have to go down to 0603 or even MELF (which is fine, except i'm not sure about their sonic properties). Anyway, i'm abandoning the plan to design and built an actual PCB with this technique but for sure i'll cook something up on veroboard with my favorite MF12 metal films, propably a 16 step one, just to hear if it's any good.
So that's the progress so far with the Audiophile pre amp, i thought i'd give you an update.
Cheers
moving to other things. I've spent quite a bit of time reading and reading threads related to subject in this forum and others.
The Lightspeed or in general preamps with LDRs seem to be in common agreement by everyone that perform better than anything... it could well be true, honestly haven't tried one yet, but i'm for sure going to try them out! Just ordered a few of them.
I've been following the Lightspeed thread and i've seen that no one (at least from the forum) has taken the difficult path to actualy make the best of them integrating some digital and software control. So as my expertise (and my job) is with this kind of things, i think i'm gonna give it a try, a few DACs and ADCs, a nice little micro, calibration routines etc. No need to match them and absolute control. So that's what i started working on.
But before i got to that conclusion i was looking how best to implement a stepped attenuator. I've read whole trains of documentation, forum threads etc. and my conclusion is that possibly the best way is using iCMOS switches. So i started working on that design as well, did the prelim schematic, started looking for the components.. and that's where i got stuck!. The cost to build a 64 stepped attenuator using good quality components and ICs is more than 200£ only the components to start with plus the cost of the PCB. The other problem i realised is that the signal path length is going to be long, so some tricks are required on the layout to actually keep it on the shortest possible length, after all 64 +1 resistors are a lot! The plan was to use 0805 thin film 15ppm 0.1% resistors, but i think to get it on a proper size and make the signal path acceptable will have to go down to 0603 or even MELF (which is fine, except i'm not sure about their sonic properties). Anyway, i'm abandoning the plan to design and built an actual PCB with this technique but for sure i'll cook something up on veroboard with my favorite MF12 metal films, propably a 16 step one, just to hear if it's any good.
So that's the progress so far with the Audiophile pre amp, i thought i'd give you an update.
Cheers
Carl_Huff said:
That circuit is a preamp, with gain and global negative feedback. JFETs are used in common-source arrangement.
If a unity-gain buffer is wanted Nelson Pass' B1 JFET buffer is a better solution:
http://www.passdiy.com/pdf/B1 Buffer Preamp.pdf
I got a question regarding the discrete buffers/pre-amps.
I've read all the positive comments about the B1, and granted it's simplicity seems an excellent buffer, but!
Isn't it better to avoid caps in the signal path?, the B1 is a single supply buffer so the caps and biasing are required. Compared to a single pair of JFETs from dual supplies how can the B1 be better? or is it better?
Cheers
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