chrissugar
Well-known member
to DC servo or not to DC servo, this is the question.
In the last few years I read tons of posts about people prefering or not DC servoed audio circuits. I remember at the old place there was a thread dedicated to this subject and there were so many contradictory afirmations that I thought this is something worth to be investigated. The opinions are divided between those who think that DC servo is a bandaid because it eliminates the coupling capacitor but has other problems (sound unnatural, constrained, has problems with low end etc, etc) and the ones who think that DC coupling is better because it eliminates the capacitors (low freq phase problems, dielectic absorbtion that smears the sound, etc)
One of the recent threads, where one member was expressing his not liking DC servos ( quote: Question is, what are remedies for that beside DC servo (which I tend not to like for nontechnical reasons) ") reminded me that this is an unsolved issue, and nobody did a serious controlled test, and draw the conclusions about this subject (or at least didn't make the results public).
So I decided to do myself this test. In fact it is a more extended test because I'm also interested not only about DC servo but also about the claims related to the audibility of various coupling capacitors.
Before anything I decided to do my tests with two different discrete opamps, with and without DC servo. The first opamp is jfet opamp based of Fred Forssell's paper, and the second is based on the JML Hybrid (which is a circuit similar to some used in some expensive comercial devices). The advantage with the Hybrid is that is has a DC servo circuit that can be activated or not depending on application. For the Jfet opamp I had to build a separate one, so I used the DC servo circuit from Fred : http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF
The first setup was built with four Hybrid type opamps as pairs of circuits for stereo evaluation, one pair with DC servo and one pair without.
The only way to make a correct evaluation is to use a coupling capacitors for each setup, because the one without servo can't be DC coupled to the next device. To reduce as much as possible the sound alteration induced by the coupling caps, I used some very high quality polyprops (MITCAP) so if there is any difference between the servoed and non servoed circuits it should be below the damage induced by the capacitor.
Anyway, based on how radical were the opinions about the sound of servo versus non servo, I was expecting some serious differences.
The setup: computer playing various test files for calibration, and 24bit 96Khz mastertape quality records, minimalist, two microphone orchestral, choir and jazz music. The computer connected to Lavry DA10 converter, the analog outs distributed two the two pairs of discrete opamp based gain blocks (one with servo, the other without). The outs of each pair of the gain blocks into the coupling capacitors and then into the inputs of the Cranesong Avocet. The Avocet was used as an A/B test device because it has a very transparent audio path and the switching is relay controlled.
For the first test I made both pairs of gain circuits as 20dB noninverting circuits, all four opamps configured in class A (LED biasing). I opted for the 20dB gain and not unity gain to make the opamps "work harder". Also everything was calibrated with the RME Totalyser to less than 0.1dB, to avoid the chance to draw wrong conclusions because of any level related problems. So the resistors in the gain blocks were perfectly matched to obtain this precission.
The audio test. I played lots of materials to be sure that my conclusions are correct. I forgot to mention, that my choice for using pairs of circuits and not mono chanels, was that in mono you can make judgements related to timbral changes, but only in stereo with coerent materials you can hear if there is any change in space definition because of phase distortion or other issues.
After listening for an hour and switching from servo to non servo and back for at least 100 times, I can say that I couldn't detect any difference in sound between the two situations. So if there is any difference in sound between a servo and not servo circuit it should be under the distortions introduced by an expensive polyprop capacitor. I'm a bit shocked by the results considering how radical opinions were expressed related to this subject.
Later I did three more tests.
The second test was with the jfet opamps in the same circuit, and had the exact same conclusions. No difference in sound between servo and non servo.
The third test was to use four identical mic preamps with the same test setup. The line level signal was distributed in pairs, and attenuated 40dB before the Lundahl input transformers. everything was calibrated like in the first test. First pair (A) was non servo and the second pair (B) was with servo.
At this test I finally could hear a difference in the width of stereo image at high frequency. Interesting was that the servoed version sounded a bit wider than the non servo but without any timbral change. Before drawing any conclusion I swapped the gain blocks between the pairs, now pair a is servoed and pair B is non servo. The patern of sound was exactly the same but this time the non servo sounded a bit wider. I can say it without any doubt that the small width difference I heard was because of the input transformers, and there is no difference in sound between DC servo and non servo.
Later I redid the same mic preamp test, this time with the Jfet opamps and had the exact same conclusion. The B pair sounded a bit wider (but timbraly identical) regardless of servo or non servo.
So my final conclusion is that there is no audible differece between gain blocks with DC servo and no DC servo. Also I think that direct, capacitorless coupling (with DC servo circuit) has the cleanest audio because there are no artifacts induced by the DC servo circuit and there is the advantage of eliminating a coupling cap.
I sugest anyone who can do this test in controlled condition to do it to confirm my foundings.
chrissugar
P.S. The next test I intend to do, is to make a setup with a gain block with DC servo, and couple it's output to the next stage directly or with various type of capacitors (polyprop, polystiren, elco), to evaluate how much damage produce various capacitors compared to direct coupling. But this will be in another day, I hope in the very near future.
In the last few years I read tons of posts about people prefering or not DC servoed audio circuits. I remember at the old place there was a thread dedicated to this subject and there were so many contradictory afirmations that I thought this is something worth to be investigated. The opinions are divided between those who think that DC servo is a bandaid because it eliminates the coupling capacitor but has other problems (sound unnatural, constrained, has problems with low end etc, etc) and the ones who think that DC coupling is better because it eliminates the capacitors (low freq phase problems, dielectic absorbtion that smears the sound, etc)
One of the recent threads, where one member was expressing his not liking DC servos ( quote: Question is, what are remedies for that beside DC servo (which I tend not to like for nontechnical reasons) ") reminded me that this is an unsolved issue, and nobody did a serious controlled test, and draw the conclusions about this subject (or at least didn't make the results public).
So I decided to do myself this test. In fact it is a more extended test because I'm also interested not only about DC servo but also about the claims related to the audibility of various coupling capacitors.
Before anything I decided to do my tests with two different discrete opamps, with and without DC servo. The first opamp is jfet opamp based of Fred Forssell's paper, and the second is based on the JML Hybrid (which is a circuit similar to some used in some expensive comercial devices). The advantage with the Hybrid is that is has a DC servo circuit that can be activated or not depending on application. For the Jfet opamp I had to build a separate one, so I used the DC servo circuit from Fred : http://www.forsselltech.com/schematics/JE16%20Mike%20Preamp.PDF
The first setup was built with four Hybrid type opamps as pairs of circuits for stereo evaluation, one pair with DC servo and one pair without.
The only way to make a correct evaluation is to use a coupling capacitors for each setup, because the one without servo can't be DC coupled to the next device. To reduce as much as possible the sound alteration induced by the coupling caps, I used some very high quality polyprops (MITCAP) so if there is any difference between the servoed and non servoed circuits it should be below the damage induced by the capacitor.
Anyway, based on how radical were the opinions about the sound of servo versus non servo, I was expecting some serious differences.
The setup: computer playing various test files for calibration, and 24bit 96Khz mastertape quality records, minimalist, two microphone orchestral, choir and jazz music. The computer connected to Lavry DA10 converter, the analog outs distributed two the two pairs of discrete opamp based gain blocks (one with servo, the other without). The outs of each pair of the gain blocks into the coupling capacitors and then into the inputs of the Cranesong Avocet. The Avocet was used as an A/B test device because it has a very transparent audio path and the switching is relay controlled.
For the first test I made both pairs of gain circuits as 20dB noninverting circuits, all four opamps configured in class A (LED biasing). I opted for the 20dB gain and not unity gain to make the opamps "work harder". Also everything was calibrated with the RME Totalyser to less than 0.1dB, to avoid the chance to draw wrong conclusions because of any level related problems. So the resistors in the gain blocks were perfectly matched to obtain this precission.
The audio test. I played lots of materials to be sure that my conclusions are correct. I forgot to mention, that my choice for using pairs of circuits and not mono chanels, was that in mono you can make judgements related to timbral changes, but only in stereo with coerent materials you can hear if there is any change in space definition because of phase distortion or other issues.
After listening for an hour and switching from servo to non servo and back for at least 100 times, I can say that I couldn't detect any difference in sound between the two situations. So if there is any difference in sound between a servo and not servo circuit it should be under the distortions introduced by an expensive polyprop capacitor. I'm a bit shocked by the results considering how radical opinions were expressed related to this subject.
Later I did three more tests.
The second test was with the jfet opamps in the same circuit, and had the exact same conclusions. No difference in sound between servo and non servo.
The third test was to use four identical mic preamps with the same test setup. The line level signal was distributed in pairs, and attenuated 40dB before the Lundahl input transformers. everything was calibrated like in the first test. First pair (A) was non servo and the second pair (B) was with servo.
At this test I finally could hear a difference in the width of stereo image at high frequency. Interesting was that the servoed version sounded a bit wider than the non servo but without any timbral change. Before drawing any conclusion I swapped the gain blocks between the pairs, now pair a is servoed and pair B is non servo. The patern of sound was exactly the same but this time the non servo sounded a bit wider. I can say it without any doubt that the small width difference I heard was because of the input transformers, and there is no difference in sound between DC servo and non servo.
Later I redid the same mic preamp test, this time with the Jfet opamps and had the exact same conclusion. The B pair sounded a bit wider (but timbraly identical) regardless of servo or non servo.
So my final conclusion is that there is no audible differece between gain blocks with DC servo and no DC servo. Also I think that direct, capacitorless coupling (with DC servo circuit) has the cleanest audio because there are no artifacts induced by the DC servo circuit and there is the advantage of eliminating a coupling cap.
I sugest anyone who can do this test in controlled condition to do it to confirm my foundings.
chrissugar
P.S. The next test I intend to do, is to make a setup with a gain block with DC servo, and couple it's output to the next stage directly or with various type of capacitors (polyprop, polystiren, elco), to evaluate how much damage produce various capacitors compared to direct coupling. But this will be in another day, I hope in the very near future.
