Swapping opamps on Gilmore Dynalo?

[Illah]

So I have a MisterX dynalo with OPA227P opamps in them. I also have AD843 opamps on hand, and according to Tangent's site these are equal or better than the OPA627. In other words, they're supposedly way better than the OPA227P. SO, since the opamps are DIP socketed it's an easy change. Before I swap it out, will this even have any effect on the sound? I think the opamps are just buffers in a dynalo and don't actually amp the sound.

Excuse my ignorance, but I'd love to know if this is a possible upgrade

--Illah

 

[robzy]

Hehe...

Opamps dont only amplify audio or act as a current buffer. They do many fantasmic jobs.

One of those involves keeping DC Offset down. It is not in the signal path at all, infact chances are you could remove them and your amp would still be going strong (i DO NOT recommend doing that as i am not really sure how the opamps are connected )

So there would be no sound difference at all by swapping opamps.

Rob.

 

[bg4533]

Quote:

Originally Posted by Kevin Gilmore (Headwize Dynalo specs) My servo feeds back to the current sources for the dual FETs in stage 1. Like all servos, it is an integrator. Due to the large (relatively) integration capacitor and the 1 meg resistor, the frequency of this filter is 0.05 Hz. With even a decent opamp, the servo's noise is in the tens of microvolts, and does not affect the operation of the current sources significantly. The servo opamp in this amplifier measures the DC at the output, if any, integrates it and applies it to the midpoint of the two LEDs. The LEDs do have a slight change in voltage with respect to current, about 3 or 4%, and that is enough to make the servo work. Notice that if the transistors or the resistors are very poorly matched, the servo will not work because its total control range is at most 10%. Most standard servos (such as the Mark Levinson or Krell servos) have a much wider range. For high impedance headphones, a little DC will not hurt the phones. With the low impedance Grados, even 0.1VDC over a long period of time will definitely damage and/or change the sound. If all the parts are hand matched, the power supplies are exactly the same and all the resistors are really good quality, the amp should be stable and should not drift. In that case, the servo could be omitted or replaced with a 20K trimmer pot wired from +16VDC to -16VDC, with the wiper going to the DC adjust pin. The prototype uses 0.05% tolerance resistors, and I hand-matched the transistors. The output DC is less than 6mV and has stayed absolutely stable for the few months I have had the unit.

Basically DC offset is bad. The opamps in the Gilmore Dynamic measure the DC offset in the amps output and output a signal opposite of the DC to cancel it out. As long as they are stable they have extremely little affect on the sound. The OPA227 was chosen because it works well in this spot, just leave it.

 

[Pars]

Quote:

Originally Posted by bg4533 As long as they are stable they have extremely little affect on the sound.

I'd change that to absolutely NO effect on the sound

 

[Illah]

That's what I thought, thanks

--Illah

 

[Garbz]

Opamp rolling in a Dynalo

Well actually you can. I did. After I found that my DC offset didn't settle below 7mV I did a bit of playing, and accidently burnt out an opa227. So i replaced it with and opa134 because it was on my desk at the time and volah, that channels offset droped below 0.8mV. Chaning the opamp on the other channel to opa134 did the same thing.

Someone gave an explaination of why a while ago but I forgot what it was.

 

[amb]

Quote:

Originally Posted by Garbz Chaning the opamp on the other channel to opa134 did the same thing. Someone gave an explaination of why a while ago but I forgot what it was.

Since this is a DC servo, the opamp's own intrinsic DC offset plays into the equation. The OPA134 has lower input bias current than the OP27 or OPA227 (by approximately three orders of magnitude), is thus better for this purpose.

 

[Pars]

Yeah, I ran into the same thing. Justin (headamp) used to use TL071s or at least had em on his parts list when he sold the boards seperately. On the dgardner board I built, I put in OPA227s thinking these must be better... not. The offset would start to go negative on one channel and the opamp would not compensate for it. I put in the TL071s from my V2 board, and no more problem.

 

[steinchen]

with op27 I got the offset below the resolution of my DMM (reading zero offset), with opa227 I get about 1mV offset, haven't tried the TL071 yet since I'm very satisfied with the op27

 

[Garbz]

Quote:

Originally Posted by amb Since this is a DC servo, the opamp's own intrinsic DC offset plays into the equation. The OPA134 has lower input bias current than the OP27 or OPA227 (by approximately three orders of magnitude), is thus better for this purpose.

Exactly what you told me the first time around thanks

Question would this also effect the accuracy of your epsilon12 circuit? I just put an opa227 in there. But I guess that the protection circuit should really care about 2-10mV, it's when it gets hairy that it trips.

 

[amb]

Quote:

Originally Posted by Garbz Exactly what you told me the first time around thanks Question would this also effect the accuracy of your epsilon12 circuit? I just put an opa227 in there. But I guess that the protection circuit should really care about 2-10mV, it's when it gets hairy that it trips.

Using a OPA227 instead of a TL081 in the ε12 protection circuit may slightly influence the trigger point, but not a big deal as that trigger point was sort of arbitrarily chosen anyway. If you happen to have a OPA227 to use, fine. There is no reason to "upgrade" from a TL081 to anything better in that circuit, though.

 

[Illah]

Quote:

Originally Posted by amb Since this is a DC servo, the opamp's own intrinsic DC offset plays into the equation. The OPA134 has lower input bias current than the OP27 or OPA227 (by approximately three orders of magnitude), is thus better for this purpose.

I have OPA134's on hand also - so you suggest they would be better? I'm familiar with DC offset but not totally sure what it means, could it have anything to do with one channel being lounder than another? Because sometimes, and it's very subtle, I feel like the left channel is louder. I always just assumed it was my ears, like the hearing is better on that side or something.

How would I test the offset? I have a multimeter, just need to know where to measure from

--Illah

 

[Pars]

DC offset measurement: set your DMM to the lowest DC volts setting (millivolts if it has it). Measure from left hot to ground, right hot to ground at the output (headphone) jack. For phone plugs (either mini or 1/4") the tip is left, ring is right and the sleeve is ground. Low (as in 0.0V) is good. Since a Gilmore is a DC coupled amp, you ideally want to short the inputs to ground for this measurement... at the very least turn the pot all the way down, nothing playing at the time.

 

[Garbz]

Audio is an AC waveform. DC offset is the DC componnet in that AC waveform.

Ideally say a sinewave would swing from -1 to +1v however if you add in 0.5v of DC into the signal it becomes -0.5v to +1.5v

While this doesn't effect sound well it would in extreme cases however because DC heats up the coils of the transducers you'll be likely to burn out headphones or speakers long before the DC effects the sound.

In a nutshell DC offset is BAD. The OPA134 reduced my offset more then the OPA227 so if you have both on hand then use the OPA134. Mind you the OPA134 makes a better amplifier so you may want to keep it for later.

 

[Illah]

Well the DC offset was pretty minimal on both opamps. Is it just me or does the volume sound louder with the OPA134? Is that even possible?

Thanks,

--Illah