My Yamaha M-60's and M-80 have 2-5 mV's offset on all of them on the right channel. Left channel is 2-3 mVWell, the thing is that they both showed 2mV before reworking them, and 2mV after.I have never seen an amp before that only had 2mV DC offset but I have also never before had an amp that had that DC servo thingy in it.
I haven't started working on the 3rd one yet. That probably won't happen until after Summer. Got too many other projects that got backed up by this one.
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Whew! No magic smoke. Was the DC offset affected?Today I replaced IC501 with an OPA1656. Measured the supply voltage afterwards and they are still at +/-15.1v, so I think that it is stable. Rechecked the idle current afterwards also. The first alp was a bit down @7mv, the 2nd amp still nominal at 8mV.
Eventually, when I get the 3rd amp repaired, I will take the 1st one and retrofit one of those 25 turn trimpots in it. The turn ones are little better than the original ones, very touchy to adjust. The 25 turn potis are a dream to adjust.
I did not replace IC502. Wouldn't want to do both of them at the same time, And I am not seeing where that op-amp is directly influenced by the signal. Someone with more insight is welcome to comment on this please, but I am a bit skeptical that there is anything to be gained here.
Sound check in about 3 hours.
James
Forgot to check it. But the DC servo op-amp is the one that I did not change, just the one in the idle current section. I didn't expect any smoke, unless it went into crazy oscillation.
One amp has 1µF film caps there, the other elcos. Both amps are behaving nicely. I think that it doesn't matter.
I have a pile of them that you can have for free if you want to pay for post.I guy could probably sell the old binding posts on the buy & sell sites for good beer money. There's a few loons out there using the old ones, or replacing the broken ones, like me for instance.
Yes.
My main goal with the PMA-737 was/is to see if a more precise (and suitable) opamp as DC servo could reduce even further the DC offset.
The "rough" M5218P keeps it around 5mV, not bad. But I thought a newer modern opamp, being in the signal path, with lower noise, Fet/Cmos input, and low Icc consumption could do it better.
The selection was made around the bandwidth and slew rate (not too far from the original) but with lower noise and lower Icc opamps to avoid instabilities.
I want to try out the other more modern ones at the bottom of the list, but they come in SMD parts, so I have to adapt them to PDIP package first.
Nice work with these beautiful POAs...
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Good to hear!Hey Karl, I have to admit to being lazy and I haven't looked for the SM for your amp to compare, but it has been 5 days now since I replaced the NJM082BD in IC501 (idle current section). The amps sound decidedly better with the OPA1656. I am going to wait a few weeks before trying something in the IC502 M5218P position.
But I can highly recommend the 1656 for IC501.
James
It also has a very high open loop gain, 150db, and 100mA current output.Good to hear!
The excellent OPA1656 is a mini beast (53MHz bandwidth - 24V/μs slew rate), and only 2.9nV on the noise parameter. Excellent Opamp.
Let us know if the DC offset is minimum and stable...
Yes, I believe it's designed for headphone amps, along with one of my favorites, the OPA1642 (5.1nV / 11MHz / 20V/μs / Icc 1.8 ~ 2.3mA (p/ch) / Fet input)
Thanks man. I caught the flu in op amp school and missed some info. Good to know brotha.
Karl, I notice that you like to use the NLA OPA2107AP.
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Hi James. I have two of them as well in my stock, thanks. They are great for line stage preamps and DAC output. They are now probably discontinued, but still a wonderful chip.Karl, I notice that you like to use the NLA OPA2107AP.
I still have 2 of them that I don't think that I will ever use. If you would like to have them send me a PM/Conversation and I will send them to you.
James
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