API style eq (MAP3100) how fine tune output level

[barbaroja]

Hi. Got one of these rare gems. Sounds absolutely fantastic. I would like to tweak the output signal level. The opamp inside has a pin named "trim" as the 2520 does. I suppose that it is intended to well, trim the signal. How to use it? What signal is it expecting? A resistor to ground? A trimpot maybe?

I was hoping somebody that knows an API style EQ (550?) may help.

 

[3nity]

It should be a 325 style line amp..there's a gain set resistor..you find a schematic or trace the schematic

 

[Bo Deadly]

Hi. Got one of these rare gems. Sounds absolutely fantastic. I would like to tweak the output signal level. The opamp inside has a pin named "trim" as the 2520 does. I suppose that it is intended to well, trim the signal. How to use it? What signal is it expecting? A resistor to ground? A trimpot maybe?

No, "trim" on the op amp itself is definitely not trimming the signal level. It's probably output offset trim. Don't mess with that. You could end up putting DC through the transformer which would cause distortion or worse.

There is probably a resistor on the - input of the op amp that will set the gain. But without a schematic it's difficult to say.

 

[gridcurrent]

I would like to tweak the output signal level. The opamp inside has a pin named "trim" as the 2520 does.
I was hoping somebody that knows an API style EQ (550?) may help.

the "trim" pin is output offset, as in a 2520.
the pin connections of the MAP 3100 are the same as an original API 550a equaliser (unbalanced input),
with the possible exception of pin 11.
pin 11 on the 3100 is labeled "input hi, 12 dB gain"
pin 11 on a 550a is gain adjust, per resistor table in the documentation.

 

[r2d2]

Hi. Got one of these rare gems. Sounds absolutely fantastic. I would like to tweak the output signal level. The opamp inside has a pin named "trim" as the 2520 does. I suppose that it is intended to well, trim the signal. How to use it? What signal is it expecting? A resistor to ground? A trimpot maybe?

I was hoping somebody that knows an API style EQ (550?) may help.

any image of the back side of pcb ?

 

[TubeMonkey]

Hey Guys,

Just picked up a pair of these.... there is a set/ knock down resistor on the input... I removed both of them and replaced with an 18k resistor off of pin 10. This got the whole unit fairly close to unity... though if you wanted to dial in exactly you could put a 25k trim pot there and dial it in. I'm currently having an issue with high frequency oscillation in the high frequency band of the EQ... which also seems to collapse the signal if the high shelf switch is engaged.... throwing up the Hail Mary hoping that a schematic exists out there or there is something similar in Melcor territory, otherwise this might be a candidate for reverse engineering and drawing something up by hand.

EB

 

[mjrippe]

It should be very similar to an API 550-A-1 but with more frequencies.

 

[FIX]

Rare indeed... The EQ is similar to a 550A, not an A1, as the A1 was years later. The TRIM pin was related to all op-amps made in that pinout configuration, which was an instrumentation amp for analog computers, servo controls, etc. It is to TRIM the DC offset, and not used in any of the audio applications.

The gain trim resistor was for use as a fader driver or fader make up amp, it gave you up to 12 dB of gain, so you could have a preamp drive the fader, then the fader at zero position would be 12 dB down, and the EQ would make that up. Leave it off and it will be close to unity.

Their will be a little gain, as they were all designed to be loaded with a 600 ohm resistor or a 600-1K fader.

For the oscillation issue, two things. Put a 33pf cap across the output op-amp between - and OUT. also, do the same for the ICs. I don't see any caps around them. The filters are either all in series between the input and output amps, (most likely the amp in the corner input.) that does the cut, then you bypass that and move the filter into the feedback path of the amps for boost. That is where there is instability. The ICs serve as buffers between the filters so they are driven by a lo-Z source.

 

[mjrippe]

Thank you Paul, I mis-read my own notes. 550A is correct, the A-1 is best forgotten

 

[FIX]

The A-1 wasn't a bad EQ, it was like a 560, but with 3 bands... The problem was that everyone "thought" (hoped) (expected) that it would be a 550A, which it wasn't at all... And hopefully it will be forgotten, but I see them listed for sale now as a classic EQ, knowingly not the 550A and they get good money. Ugh...

 

[TubeMonkey]

Thanks for all the info Paul. I was able to patch it up without adding any caps…. The oscillation issue was caused by a missing resistor in the feedback loop of one of the 4558’s. It was an old carbon resistor that had cracked and split. The second issue turned out to be a bad 4558 outputting DC on one of the amp outputs.

now that I got this one up and running I discovered the other one is having a similar problem where one of the 4558’s is outputting -12vdc and collapsing the signal… this one however isn’t a bad IC as it happens with every amp I put in there. Also the -12vdc on pin 7 goes away when I pull it.

 

[Bo Deadly]

now that I got this one up and running I discovered the other one is having a similar problem where one of the 4558’s is outputting -12vdc and collapsing the signal… this one however isn’t a bad IC as it happens with every amp I put in there. Also the -12vdc on pin 7 goes away when I pull it.

Sounds like a broken feeback path like a feedback path resistor is broken inside. If you nudge it you might even see it crack open.

 

[TubeMonkey]

Just realized I never followed up on this....

Ended up figuring out the other EQ. It turned out the person who had this previous to me inserted a dual amp where there should have been a single. Since there is no schematic available it was probably easy for someone to think that all the band buffers were dual amps when in fact one was a single... Figured it out by looking at the power ingress on each chip.

 

[Ike Zimbel]

The A-1 wasn't a bad EQ, it was like a 560, but with 3 bands... The problem was that everyone "thought" (hoped) (expected) that it would be a 550A, which it wasn't at all... And hopefully it will be forgotten, but I see them listed for sale now as a classic EQ, knowingly not the 550A and they get good money. Ugh...

Hi Paul,
I have had a few "550M" modules through the shop. These are NOT the "Mastering" version, but a very old 550. No one at API was able to tell me anything about them, but no one at API was there when the company started. Do you know anything about them? I may have some pics but I will have to dig for them on another computer.

 

[FIX]

Send a pic or two...