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api 550m
Old 10th April 2012
  #1
Here for the gear
 

api 550m

I picked up a pair of these thinking that I was getting a pair of older 550a's... Are these identical to the 550a's but 1/2 db steps? I believe that these were made in the mid 80's.

I can't find much info on these... Without opening them up are they 2 2520 op and an output transformer?

Any info would be great.

I would be using them mostly for tracking and individual tracks while mixing rather than the 2 bus (i think at least...). Is 3db of gain of boast enough for non mastering?

thanks
Old 10th April 2012
  #2
Gear Guru
 
AllAboutTone's Avatar
 

I think those are mastering EQs, not sure. They maybe the older 7600
Old 10th April 2012
  #3
Lives for gear
 
JonMiller's Avatar
How do they sound?

I'm a firm believer API has never made a bad piece of gear.

Just like how Bill Murray has never made a bad movie.
Old 10th April 2012
  #4
Gear Guru
 
AllAboutTone's Avatar
 

Quote:
Originally Posted by JonMiller View Post
How do they sound?

I'm a firm believer API has never made a bad piece of gear.

Just like how Bill Murray has never made a bad movie.
I almost bought a couple a while back, I don't know what they sound like.
google 550m they are a ton of stuff on them even GS threads.
Old 10th April 2012
  #5
Lives for gear
 
JonMiller's Avatar
Ooooo I have seen these.

I believe the API 5500 replaced them.
Old 10th April 2012
  #6
Quote:
Originally Posted by JonMiller View Post
How do they sound?

I'm a firm believer API has never made a bad piece of gear.

Just like how Bill Murray has never made a bad movie.
Word!
Old 10th April 2012
  #7
Pretty sure they're just the 550b's but with finer increments for mastering purposes.
Old 10th April 2012
  #8
Here for the gear
 

After a little closer look, having placed them in the lunch box... but haven't patched them or listened to them yet, these are the original 550's with only 3 frequencies on the Hi band (stops at 15).

They also have the longer frequency knobs but the model (on the side engraved plate) is a 550m with serial numbers in the 1400's.

I am guessing that they might simply be the original 550s but odd that they say 550m.

I guess I will plug them in tomorrow and see if they are 1/2 db steps.

thanks for the replies
Old 10th April 2012
  #9
the common API 550m is a 550b with half db steps
high freq has 7 positions tho'
Old 10th April 2012
  #10
Lives for gear
 
Ike Zimbel's Avatar
 

Quote:
Originally Posted by Currie View Post
After a little closer look, having placed them in the lunch box... but haven't patched them or listened to them yet, these are the original 550's with only 3 frequencies on the Hi band (stops at 15).

They also have the longer frequency knobs but the model (on the side engraved plate) is a 550m with serial numbers in the 1400's.

I am guessing that they might simply be the original 550s but odd that they say 550m.

I guess I will plug them in tomorrow and see if they are 1/2 db steps.

thanks for the replies
I had a pair of these in the shop last year, also with s/n's in the low 1400's. I couldn't find out much about where the "M" designation came from, but they are quite old. I would say early-to-mid 70's build. Both units needed to be re-capped, and one had extensive heat damage on the circuit board where the transistors for the Eq amps live. I ultimately had to build a little sub-board and mount those components separately (which is what API ended up doing in later versions). The attached pic is the finished product.
Best,
Ike
Attached Thumbnails
api 550m-550m-eq-amps.jpg  
Old 10th April 2012
  #11
Here for the gear
 

Ike,

These are probably them as I am also in Toronto... bought from Rich

Good to know another tech here in the city.
Old 9th May 2012
  #12
since there are some tec's on this page, would u say for a first time api eq buyer to go with a pair of new api 550b's or these older 550 pair from the 80's, im just an hour from toronto, what work may need done on them? thanks slutz!!

2- API 550 Equalizers - City of Toronto Musical Instruments For Sale - Kijiji City of Toronto Canada.
Old 11th May 2012
  #13
Lives for gear
 
Ike Zimbel's Avatar
 

If the ad is accurate, they shouldn't need any work done.
Best
Ike
Old 11th May 2012
  #14
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shortstory's Avatar
If anything, it looks like a 550A with fewer frequency options; namely- no 5k or 12.5k on top- and no 300 & 400 on the bottom- which is fine if you're OK with that. Not sure I see the 1/2 db increments I thought you refered to.
Old 11th August 2012
  #15
Lives for gear
 
Ike Zimbel's Avatar
 

So, I have another pair of these "550M" units in the shop and after doing some reading, I would say that they are in fact the original "550" equalizer. If you read through the attached .pdf, you will see that it starts out calling itself the "API 550 Equalizer", but by page three is about the "550 A", and has a paragraph on interchangeability with the "original 550 Equalizer".
These "550M" units in the s/n1400 range, have the incandescent tally (In/Out) lamp wired to terminals 6 & 7 on the edge connector, rather than the integrated LED lamp of the 550A. Now, having said that, the circuit board has "550-B" etched on it...
Cheers,
Ike
Attached Files
File Type: pdf API550.pdf (643.2 KB, 277 views)
Old 11th August 2012
  #16
Gear Maniac
thanks for posting that schematic, I have an early 550 shipping to my studio next week. Interesting to see that overall gain can be adjusted with a single resistor (91R between terminals 8 and 11 for 6dB gain), which I might add to compensate the gain loss from the unbalanced input, driving it with transformerless output stages (i.e. DA converter).
It would be interesting to see a schematic of the current 5500 so see how they made the gain range switchable 2dB / 1dB / 0.5dB, maybe this can be incorporated into the vintage modules, at least a 2dB / 1dB option would be nice to have.
Old 18th October 2013
  #17
Gear Maniac
ok, troubleshooting my aquired 550 I ran across some information that may be of interested. The schematic posted above is not for the original 550, it seems to be an early 550A schematic, which is similar with some differences:

The 2M2 resistors are not present on the original circuit (AP-550C in my case), but adding these for the 2520 and buffer amps might increase stabilty and I have seen it done on a picture posted on this thread:
https://www.gearslutz.com/board/9311857-post25.html

There are only two buffer amps (not four) and they are implemented straight on the main PCB, not on auxiliary PCB's as denoted by A3, A4, A5 and A6 in the schematic posted. The buffers are implemented with 2N5087 (PNP) and TIS97 (NPN).
EDIT: it seems the buffer amps are implemented in a different fashion in the "non - A" circuit, there are six transistors in PNP/NPN pairs, making up three sections.

The TIS97 have had yellow, red, white color marking done on their back side, I have no clue as to what this means, maybe some sort of HFE matching with resistor color coding, that's just a wild guess. I'll try and replace them with 2N5088, should be close enough.
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