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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 18th October 2017, 05:06 AM   #2221
dbis is online now dbis  United States
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Salas DCG3 preamp (line & headphone)
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.
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Old 18th October 2017, 08:06 AM   #2222
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
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.
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Old 18th October 2017, 04:29 PM   #2223
dbis is online now dbis  United States
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Salas DCG3 preamp (line & headphone)
Thanks for the tip Salas. That’s a lot easier.
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Old 24th October 2017, 02:23 AM   #2224
drpro is offline drpro  United States
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Salas DCG3 preamp (line & headphone)
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
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Old 24th October 2017, 05:38 AM   #2225
ammel68 is offline ammel68  United States
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Salas DCG3 preamp (line & headphone)
Quote:
Originally Posted by drpro View Post
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
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.
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Old 24th October 2017, 06:12 AM   #2226
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
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
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Old 24th October 2017, 02:22 PM   #2227
drpro is offline drpro  United States
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Salas DCG3 preamp (line & headphone)
Thanks for the insights Ammel. I am more of a builder than one who understands the nuances/differences of the circuits. My pleasure comes from the build and enjoyment of listening to the music.
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Old 26th October 2017, 07:42 AM   #2228
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
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.
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Old 26th October 2017, 08:24 AM   #2229
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Salas View Post
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.
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?
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regards Andrew T.
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Old 26th October 2017, 09:31 AM   #2230
Salas is offline Salas  Greece
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Salas DCG3 preamp (line & headphone)
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.
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