Thanks Salas,
I have the Tea-bag kit so I believe there are a couple 7.5R resistors included for high bias. I'll try those. I'm using a 3U full width chassis, so not too crowded. I didn't check the temp but my aluminum bar didn't feel that hot with the lower bias. Should I remove the op amp before powering back on at the higher bias?
I'll measure the output offset again, making sure the black probe is to ground.
Thanks again.
I have the Tea-bag kit so I believe there are a couple 7.5R resistors included for high bias. I'll try those. I'm using a 3U full width chassis, so not too crowded. I didn't check the temp but my aluminum bar didn't feel that hot with the lower bias. Should I remove the op amp before powering back on at the higher bias?
I'll measure the output offset again, making sure the black probe is to ground.
Thanks again.
Yes, when setting another output stage bias level remove the op-amp also and trim down the offset to its unassisted best before putting the op-amp back again. Same procedure as you did before.
There is an easy way to check intermediate levels of bias heat first without having to unmount the PCB and the Mosfets or to desolder anything. Say you parallel a 47R over each installed 12R and you go to 9.6R. Or a 33R//12R takes you to 8.8R and so on and so forth. 1/4W resistors will do.
There is an easy way to check intermediate levels of bias heat first without having to unmount the PCB and the Mosfets or to desolder anything. Say you parallel a 47R over each installed 12R and you go to 9.6R. Or a 33R//12R takes you to 8.8R and so on and so forth. 1/4W resistors will do.
On a recent Mouser order I purchased a OPA1642a opamp. I have been listening to it in the DCG3 and the Pass DIY headphone amp. It is nice. A bit to bright and forward for my liking. Music I listen to seems to lack fullness and roundness.
Salas I must say you nailed it with the AD823 choice, that is my favorite opamp/sound.
Thanks
David
Salas I must say you nailed it with the AD823 choice, that is my favorite opamp/sound.
Thanks
David
The op-amp in Salas' DCG3 is used as servo to control DC offset at the outputs.
In Wayne's HA, the op-amp is used as a VAS stage.
You do realize that these are 2 totally different implementations for a dual op-amp, don't you??
You're much more likely to "hear any differences" between op-amps in Wayne's design since the audio signal is being amplified by the op-amp in that design.
The fact that Salas uses the AD823 in his design, could also have some bearing in your thinking it's also the better sounding op-amp in Wayne's design. I don't know since conclusions are easy to draw.
Just FYI...keep an open mind and realize there are a LOT of other dual op-amps that will sound better than the AD823 in Wayne's HA.
Some may be singles that you'll have to use an adapter with.
Personally, I would start with a pair of OPA627BPs on an adapter in Wayne's HA.
Having tried various dual op-amps in Salas' DCG3, I detected little if any appreciable differences in SQ or DC offset.
BTW
Headphone listening is very popular these days and we have learned a lot for this mode of DCG3 use here. We have also seen subjective feedback about DCG3's line level use with low to medium power Pass or tube amplifiers on mostly efficient enough speakers in this thread. But when wondering on how it would have been to use the DCG3 with strong class A/B or D power amps on big difficult magnetic panel speakers, such reporting was absent. So here's a translation of some posts regarding that kind of use for our richer information. Coming from a Greek forum (AVclub).
Initially with an Avondale NC200 bjt amp:
"The preamplifier shows its features on the job. Class A dynamics, mosfet consistency and unmodified sound. What I liked is the balance between volume and three-dimensional image. Also, absence of harsh in the high. I do not get into details because first I have reset the capacitor to the amplifier's input for safety, and the capacitor in the NFB has also changed, which has had a big effect on the bass (so I leave it out for the time being)" Savvas X
Later on Holton & First One Mosfet amps:
"In order to have a clearer picture of the preamplifier, I heard two mosfet amplifiers that I have for a long time and can drive Magnepan with the existing passive XO having 180 and 200W / 8Ω power reserves and an enviable damping factor (> 2000 both two). The conclusion is that the preamplifier does not alter the sound effect as the character of the two amplifiers sounds distinct. I have to add that they have different philosophy of constructions and differ in their sound as I've heard in another system. I still can not distinguish in my system some emphasis on the audio spectrum and other individual features from the DCG3 side." Savvas X
Another month later while evaluating on Hypex Class-D amps:
"This fashion that wants DACs to drive finals once must stop. Let me say that having a DAC which at the output stage includes transistors for power amps and consumes more than many of those amps I also did the attempt.
Result: Hypex Ncore driven directly from the DAC sounded slightly subpar than DAC -> DCG3 -> Hypex UCD400. And clearly the UCD400 is not better than Ncore (they have the same character though), just missing the right preamplifier. I have enough plans for finals and so much for DAC, my only plan at pre-amp level now is for the attenuator." Savvas X
Headphone listening is very popular these days and we have learned a lot for this mode of DCG3 use here. We have also seen subjective feedback about DCG3's line level use with low to medium power Pass or tube amplifiers on mostly efficient enough speakers in this thread. But when wondering on how it would have been to use the DCG3 with strong class A/B or D power amps on big difficult magnetic panel speakers, such reporting was absent. So here's a translation of some posts regarding that kind of use for our richer information. Coming from a Greek forum (AVclub).
Initially with an Avondale NC200 bjt amp:
"The preamplifier shows its features on the job. Class A dynamics, mosfet consistency and unmodified sound. What I liked is the balance between volume and three-dimensional image. Also, absence of harsh in the high. I do not get into details because first I have reset the capacitor to the amplifier's input for safety, and the capacitor in the NFB has also changed, which has had a big effect on the bass (so I leave it out for the time being)" Savvas X
Later on Holton & First One Mosfet amps:
"In order to have a clearer picture of the preamplifier, I heard two mosfet amplifiers that I have for a long time and can drive Magnepan with the existing passive XO having 180 and 200W / 8Ω power reserves and an enviable damping factor (> 2000 both two). The conclusion is that the preamplifier does not alter the sound effect as the character of the two amplifiers sounds distinct. I have to add that they have different philosophy of constructions and differ in their sound as I've heard in another system. I still can not distinguish in my system some emphasis on the audio spectrum and other individual features from the DCG3 side." Savvas X
Another month later while evaluating on Hypex Class-D amps:
"This fashion that wants DACs to drive finals once must stop. Let me say that having a DAC which at the output stage includes transistors for power amps and consumes more than many of those amps I also did the attempt.
Result: Hypex Ncore driven directly from the DAC sounded slightly subpar than DAC -> DCG3 -> Hypex UCD400. And clearly the UCD400 is not better than Ncore (they have the same character though), just missing the right preamplifier. I have enough plans for finals and so much for DAC, my only plan at pre-amp level now is for the attenuator." Savvas X
There is a high DC precision dedicated OP-AMP from Analog Devices that I was looking up the other day which is sort of an oddball by drawing very low input currents like the best JFET input ones do but its super-beta BJT technology. This approach additionally offers great stability of input currents vs temperature changes. In other words it champions the low DC drift performance aspect.
On the other hand its far slower and kinda noisier than those we have used for servo here. Since its also a normally priced double OP-AMP readily available in DIP-8 through hole package, that makes it very easy to pop it in and see what happens. We should remember to buy one in some future parts order to survey it in our context too. Its called AD706.
On the other hand its far slower and kinda noisier than those we have used for servo here. Since its also a normally priced double OP-AMP readily available in DIP-8 through hole package, that makes it very easy to pop it in and see what happens. We should remember to buy one in some future parts order to survey it in our context too. Its called AD706.
If one adopts the non-inverting topology where there is a pre-filter before the integrator and one also adds a post filter into the output resistor, then I think the slowness of this ad706 does not matter to the performance of the amplifier, nor to the performance as a good DC servo.
AD suggest the ad8622 as an alternative.
How would this one perform?
Because we already have a non inverting & pre filter servo topology here, that is why I was looking it up since slowness should not be too critical. I have tried DC precision labeled slow ones before in this but they had normal BJT input currents and they performed not too well, primarily because our pre filter has very high impedance.
Well, the other AD one is not as practical in this situation because it comes in SOIC package only, it looks generally bit more advanced though, like it has 11nVrtHz instead of 15nV, more PSRR, more output current capability, than the 706. For input bias and input offset current they both have too close a max spec to expect a real DC precision difference in our practice I suppose. Who specifically designs around them in a lab for instrumentation gear would know, guys like us with such a multi chip accepting servo app of various temperature delta builds would not know an absolute performance answer between those two chips.
Well, the other AD one is not as practical in this situation because it comes in SOIC package only, it looks generally bit more advanced though, like it has 11nVrtHz instead of 15nV, more PSRR, more output current capability, than the 706. For input bias and input offset current they both have too close a max spec to expect a real DC precision difference in our practice I suppose. Who specifically designs around them in a lab for instrumentation gear would know, guys like us with such a multi chip accepting servo app of various temperature delta builds would not know an absolute performance answer between those two chips.
Hi Salas,
I was finally able to get back to my build yesterday. You had suggested I up the bias, since I had room in the chassis, by piggybacking the 12Rs. I just replaced the 12R's with the 7.5R's since I didn't have any 33 or 47Rs on hand. It was easy to switch them out.
After running for about 20mn I got 144.6mA and 144.1mA across R10.
Then I adjusted the DC offset as close to zero as possible with opamp chip removed. I couldn't get exactly zero; it does fluctuate a bit but I believe that's normal. After inserting the chip my final DC offset was between -1.9mV and 0mV on right channel and between +1.1mV and -0.6mV on the left. Are those numbers ok? I was careful to keep the black probe on ground so that should be the correct polarity.
After being on for about 30mn the aluminum bar heatsink was 38-40°C measured with my Extech dmm.
Thanks again,
Dan
Depends on the op-amp sample you got. Mine does not go beyond 1mV positive for instance and its seldom to go little negative if at all. But I had another sample that was bit wilder on its one half than its other half and more negative. Its their input stage manufacturing tolerances. The offset you now got is by all means benign for practical purposes.
On the other hand if you want to make an experiment, there are R12 & R13 1Meg that you can test shunting with circa 100K each if you got any. This test may work towards "tightening" the specific sample of op-amp or not.
On the other hand if you want to make an experiment, there are R12 & R13 1Meg that you can test shunting with circa 100K each if you got any. This test may work towards "tightening" the specific sample of op-amp or not.
Thanks for the quick reply Salas. Good to know my numbers are ok. Actually I would be up for an experiment but I'm not quite sure how to shunt R12 & R13 with a 100k resistor. If it's not to complicated would you mind explaining to me how to do that?
Thanks,
Dan
Well, I spent the weekend putting everything together in the Pesante 3U chassis. I had to scavenge some parts from my DCB1 since I was too impatient to order new ones. So there are still a lot of odds and ends to sort out as you can see from the picture.
However, I was able to listen with my test setup, Amp Camp monoblocks and Focal 714V speakers, and it sounded INCREDIBLE! I’m not good at audiophile descriptions so I won’t try but it’s super detailed, yet natural, with strong, deep bass. I can't wait to try it in my living room system: Sony DIY vfet amp and Reference 3A Decapo speakers.
I had bought extension kits for the stepped attenuator and rotary selector but the bushings were too short for my 10mm thick front panel so I just grabbed the whole 3mm front panel from my DCB1 and used that, expecting to have some buzz or noise with the long cables from the i-select to the attenuator on the front but there is zero noise, no humms or buzzes! And that’s even more surprising since I haven’t put in the audio ground yet. Now I have to figure out how to deal with the 10mm black front panel.
Actually, do I even need to add the audio ground if I don’t have any noise?
Thanks again to you Salas for sharing your concepts and replying so quickly to all my weird questions, and to Tea-Bag for the kits and to everybody on diyaudio.com for all the helpful advice.
P.S. Salas, I haven’t tried piggybacking the 1meg resistors to try to get a better DC offset yet, but I will.
However, I was able to listen with my test setup, Amp Camp monoblocks and Focal 714V speakers, and it sounded INCREDIBLE! I’m not good at audiophile descriptions so I won’t try but it’s super detailed, yet natural, with strong, deep bass. I can't wait to try it in my living room system: Sony DIY vfet amp and Reference 3A Decapo speakers.
I had bought extension kits for the stepped attenuator and rotary selector but the bushings were too short for my 10mm thick front panel so I just grabbed the whole 3mm front panel from my DCB1 and used that, expecting to have some buzz or noise with the long cables from the i-select to the attenuator on the front but there is zero noise, no humms or buzzes! And that’s even more surprising since I haven’t put in the audio ground yet. Now I have to figure out how to deal with the 10mm black front panel.
Actually, do I even need to add the audio ground if I don’t have any noise?
Thanks again to you Salas for sharing your concepts and replying so quickly to all my weird questions, and to Tea-Bag for the kits and to everybody on diyaudio.com for all the helpful advice.
P.S. Salas, I haven’t tried piggybacking the 1meg resistors to try to get a better DC offset yet, but I will.
Attachments
You are welcome. Congratulations 
Regardless of having some construction details to finish like rods or a couple of other minor things to check* your build is objectively already a success for proper function from what you report. Nice to know that you are subjectively happy too. As far as first impressions can go of course. Let us know what happens in your main system also. Interesting electronics and speakers there.
There's always more to discover in the long run even after we get a grip on the performance. I was listening to some Paul Weller 90s stuff today for example and as I thought I knew this preamp and that recording well, right there it sneaked on me again.
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*About your little finishing tests:
Noiseless already is nice. You could also test what happens when referencing the chassis to the signal ground with a crock clip to a mounting screw and if there is no advance in detail or smoothness or for some total system ground reason even a tiny noise comes up or it simply sounds bit worse you may very well skip it.
Piggyback on the R12s only as first test. That will move the pole frequency of the servo's input filter the other away from the main integration pole and we will see if that can help tighten the particular op-amp sample a bit or not.
By the way your stepper pot is 20K Log as recommended, yes?
Regardless of having some construction details to finish like rods or a couple of other minor things to check* your build is objectively already a success for proper function from what you report. Nice to know that you are subjectively happy too. As far as first impressions can go of course. Let us know what happens in your main system also. Interesting electronics and speakers there.
There's always more to discover in the long run even after we get a grip on the performance. I was listening to some Paul Weller 90s stuff today for example and as I thought I knew this preamp and that recording well, right there it sneaked on me again.
------------------------------------------------------------
*About your little finishing tests:
Noiseless already is nice. You could also test what happens when referencing the chassis to the signal ground with a crock clip to a mounting screw and if there is no advance in detail or smoothness or for some total system ground reason even a tiny noise comes up or it simply sounds bit worse you may very well skip it.
Piggyback on the R12s only as first test. That will move the pole frequency of the servo's input filter the other away from the main integration pole and we will see if that can help tighten the particular op-amp sample a bit or not.
By the way your stepper pot is 20K Log as recommended, yes?
Just received today a custom O-core power tranformer from China.
But manufacturer misread my specifications and sadly wound one sec. instead of two.
I have only one 18v-0-18v available in a 100w. core.
Before complaints, Salas please lets me know best option here:
-Use as it is, wiring parallel to both channels.
-Order another tr. identical to complete double mono, or
-Order new with two sec. as it should.
Thanks in adv.
But manufacturer misread my specifications and sadly wound one sec. instead of two.
I have only one 18v-0-18v available in a 100w. core.
Before complaints, Salas please lets me know best option here:
-Use as it is, wiring parallel to both channels.
-Order another tr. identical to complete double mono, or
-Order new with two sec. as it should.
Thanks in adv.