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Vint 600Ohm to Modern matching Restr 500 Series Preamps
Old 17th September 2007
Lives for gear
ulysses's Avatar
All true. So where does this other poster's 512C fall? I haven't really examined the issue with that circuit or that transformer. In my experience however, low-ratio quadfilar output transformers tend to behave extremely well under almost all conditions. I don't expect the 512C will be affected much by loading. To the extent that it is, I believe you will find the amplifier to be the cause. (not that the 2520 is lacking in output drive, but I think variance in load tolerance will be found among the various manufacturers' variations on the Discrete Op Amp theme).
Old 17th September 2007
Gear Maniac

Quote: Thethrillfactor

For the longest i could not understand why the modules in the console sounded so much better than when in the lunchbox.

The reason is the Api modules are used to seeing a different load when in the console.

Being loaded down differently affects the freq response.

In the console they are creamier and wider sounding.

In the lunch box they are thinner,brighter and a little mid foward.

This is the sonic characteristic i've seen a lot of people attach to API modules.

In talking with a number of people(including API) i've come to realize that the modules are used to seeing a different load on the output.

API recommends a 600ohm resistor.

I tried it and it wasn't as close to the sound of the console.

Again in doing research i ran into a conversation with John Klett and he said he normally sets them up with a 1200ohm resistor on the output.

Tried it and voila!!!

The closest sound to the console out of the lunchbox.

Its not exactly there but pretty close.

I suggest you try it and see for yourself.
Old 18th September 2007
Lives for gear

Are the rest of the electronics (capacitors mainly) in good condition, this will seriously inpact on the 'variation' between modules. A 1200 ohm resistor sounds like an excellent start.
Matt S
Old 18th September 2007
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ulysses's Avatar
Sorry, I meant variations between brands of similar styles of circuit, not variations between specimens. But good point about the caps - that's largely where you'll get your variations from specified performance in old gear, including an increase in output impedance.
Old 19th September 2007
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A27Hull's Avatar

Originally Posted by I.T. View Post
Again in doing research i ran into a conversation with John Klett and he said he normally sets them up with a 1200ohm resistor on the output.
concerning the 512c,

A 1200 Ohm resistor added in parallel to an input impedance of a 002 1/4" input would yield a total impedance of 1199.856 ohm, practically reducing the load from 10 Meg to 1200 Ohms. [As I've learned recently, the total R in a parallel circuit is always lower than the smallest resistor value, and I guess this translates to impedance as well.]

What is the benefit of doing this sonically. My uneducated guess is that the frequency response of the 512c would become a lot less linear, making for a kind of "sound," that some perceive better as the linear response.

Is this true?

If the next device was 600 ohms input impedance, the collective load on the 512, using a 1200 ohm resistor would be 400 Ohms. This would be "under-loading" a device that was meant for the 600 ohm impedance matching scheme. What would be the effect of "under-loading?"

If the next device (say a 525 reissue,) which has a 30 K ohm balanced input impedance, was added to by a 1.2 k ohm resistor, the resulting load would be 1153.85 Ohms. Again my guess would be that the frequency response would become less linear, and again, if not for that certain "sound," why?

I have a lunchbox with a 512c and a 525 reissue, so this subjest interests me, as I do find the API pre to be "mid forward." However, I have no experience with the consoles to compare to.

Perhaps a simple XLR barrel coupler with "impedance pad" could be an easy way to find out.

Thank you all very much for your discussion and knowledge!

Andrew Hull
Old 19th September 2007
Gear Maniac

Quote: John Klett

As far as changing amplifiers goes I have not seen this make any real difference with regard to the HF peak in the response curve. We have a drawer full of 2520 pin-compatible opamps. This pinout was originally standardized by George A. Philbrick who produced the first commercially available opamp and it was widely used. Analog Devices, Burr Brown and others making operational and instrumentation amplifiers used it, and RCA, Melcor, API, MAP, Jensen and so on picked up it naturally as a form factor for audio opamps...

I think we have tried most of these in the 512 on my bench that I use as an opamp tester... they all have the same HF peak in response from one to the next... it's opamp independent.

The termination on the output of the transformer of an api 512C or pretty much any api device with a stock output transformer has almost everything to do with the output transformer and very little to do with the amplifier that drives it. So if you remove the 2520 and slap some other opamp in there in place of it the transformer will have nearly the same high frequency peak it had with the 2520. I would say that the amplifier has absolutely nothing to do with it but actually there is some interplay, just not very much.

The output transformers api stuff uses are "low spec" iron transformers. Low spec means the specification given to the transformer winder is not extensive... they are cheap $8 transformers... maybe $10 today or $5 if you have them made in China. When running without any termination, these transformers have resonant peak up above the audio band and some termination helps. It does not have to be a 600 ohm termination. I have found that something in the range between 1200 and 2000 ohms is fine. That is enough to damp the peak out.

I go with values higher than 600 because one may patch into an 1176 or something else that has a 600 ohm input and a double 600 ohm termination would be problematic... whereas 1500 in parallel with 600 is not too bad. I often solder the termination resistor into the module near (but not on) the edge connector pins. It may well be that the 512 has a jumper that connects a termination resistor... that would have been a good thing to do as part of the revision from the 512B to the 512C.
Old 19th September 2007
Lives for gear

Thanks for the J Klett piece.
What has been forgotten is HISTORY.
The kids of today expect 'plug and play' without THOUGHT.
Equipment was designed to have 600 ohm output impedance and EXPECTED to see a load of 600 ohms. As most things had transformers it was essential for proper usage.
when transistors (then ICs ) came in, with the possibility of driving multiple loads and being more tolerant of cable characteristics people gradually forgot the need for correct termination as newer gear did not need it.
Yes, make up a 'barrel' with 680 / 1K0 or 1K5 resistors in it and try it out for yourself.
Matt S
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