LM6172 in a Pimeta

[dsavitsk]

Has anybody done this? Does anyone have any advice on how to keep it stable, keep offset low, etc?

 

[skyskraper]

ive used it in one of my old pimeta's that i gave to a friend, i had no offset problems or audible stability problems (nor any heat indication of oscillation). touchwood though. i can't give a long term report because the guy wanted more bass (ie: a flabbier sound) so in went the opa2134

 

[dsavitsk]

Was yours class a, stacket buffers, etc., or was it more stripped down?

 

[skyskraper]

class a yes, single and double stacked buffers (started off double until i made it a portable). didn't have a problem. i didnt try it without R3G however.

i say just grab some and see

 

[dsavitsk]

They are grabbed and should be here in a couple of days. Did you use 6171 in the ground channel as well?

 

[skyskraper]

yup. thats just my singular experience though, im not sure whether i was just lucky or what.

 

[dsavitsk]

I put these into my Pimeta. From Tangent's pages, it seems that one might want to add some caps close to the opamp pins, or alter some resistors, or do someting with r3g ... I didn't do any of that. I just put them in.

The chips do seem to run hot (though not necessarilly much hotter than anything else), but I am not having any oscilation (that I can detect) and there is no real offset (6mV on both channels.)

Without burn in, these chips are very clean, open, and airy sounding and they have a tremendous amount of low level detail. Interestingly, they aren't bright, it is just that there is not a lot of bass (though this could be that one needs more caps than I have -- I have 4x 680uF FM's) but the bass that is there is tuneful. To me, they sound like good, smooth solid state, which is what they are I suppose. If the opa627's sound brooding, these opamps have a sunnier disposition. All in all, a nice change and worth the $6.50 experiment.

 

[Pars]

I use these in my Rotel RCD-855 (alternately with AD826s). Last time I put them in I soldered a 0.33uf stacked film cap across the V+/V- pins on top of the chip, but did not put any additional bypassing into the player PCB. They seem to be working fine. I know Tangent says that bypassing in this manner is not as good as individually bypassing each power pin, and I'm sure he's right, but it works for me in this case. I may in the future go in and do some more, but its been running like this for a year or better, so...

 

[ericj]

10pf between output and inverting input on each device will just about entirely kill high-frequency (rf) oscillations without negatively affecting audio.

It's hard to describe what the high frequency oscillation sounds like. I breadboarded an A47 with an OPA404 the other day and it sounded vaguely odd until i bypassed the gain stages in this way. The lm617x is much faster and less stable than the opa404.

I would recommend adding these to any high speed wide bandwidth opamp just on principle.

Opamps tend to run faster and with wider bandwidth when their current load is relieved by a current buffer, and this is one of the basic reasons that headphone amps sound good - the source improves with an easier load. An lm617x is running much faster and wider in a pimeta than it would in a cmoy.

If you have mica go ahead and use it but ceramic will work just as well. No audio actually passes through this cap, it just provides a short circuit for rf noise.

The value isn't critical. 10 or so picofarads, maybe 30 being the top end.

(the opa404 (a quad) sounds very neutral and detailed without much of the 'darkness' attributed to BB parts, but the current output is minimal and thus i think for headphones it probably only works in the A47 type configuration, and then not really for grados.)

 

[jacky]

Don't know why so many people mentioned LM6172 as unstable. I have used them to mod quite a few audio equipment such as a Pioneer LD (laser disc) player, a SONY cassette deck and a Pioneer DVD player which uses the SOIC version. I think it is truly an excellent dual opamp at a good price. Much better than the 4558 they replaced and better than NE5532. On a par with the AD8620.

 

[nikongod]

Quote:

Originally Posted by jacky On a par with the AD8620.

not at all, an 8620 costs like 8 times as much money....

 

[Xakepa]

I wouldn't say a cap in the feedback loop of the opamp is "not in the signal path". Because of the way differential amp works, such cap modifies the output.

It wouldn't be in the signal path when it's by-passing the rails.

 

[Pars]

Quote:

Originally Posted by dsavitsk I put these into my Pimeta. From Tangent's pages, it seems that one might want to add some caps close to the opamp pins, or alter some resistors, or do someting with r3g ... I didn't do any of that. I just put them in. The chips do seem to run hot (though not necessarilly much hotter than anything else), but I am not having any oscilation (that I can detect) and there is no real offset (6mV on both channels.)

I'm helping my son build a Pimeta currently and he wants to use an LM6172 in it. What gain did you use in yours? We changed out R3 to 3.32K for a gain of ~4. I still have the LM6172s in my CD player at the moment, so I haven't tried them yet. I did put an AD826 in and the offset was 3.x volts (using an OPA227 in the ground channel) on both channels. Offset with an OPA2134 is ~ 1mV, so the amp itself is fine. Single buffers, no class-A bias yet.

 

[skyskraper]

i used a gain of ~4 also. i can't remember my offset number but i do remember it was not far above the standard figure (like 1 or 2 mV more) i think i got lucky.

 

[dsavitsk]

Quote:

Originally Posted by Pars I'm helping my son build a Pimeta currently and he wants to use an LM6172 in it. What gain did you use in yours?

Mine has a gian of 4. I can't report on the long term stability as the LM6172's got replaced with some AD843's which are more to my liking. I think the LM6172, while clean and resolving, are too bright. Big caps on the rails might help (I have 4x 680uF I think), but I am out of room to try.