Melcor AML-27 Preamp questions

[gang of elk]

[quote author="pmroz"].

gang of elk...
i can recommend the bauman/peter c./etc. melcor/a.p.i. project.
sounding great here.

did you do a schemo of those eqs? I'm interested in a couple 2-band eqs for 2ch bass pre. How do you like them on the low freqs?[/quote]

hi pmroz, thanks for the recommendation, i've indeed looked into a melcor pre project but have to catch up on other proj's first, but melcor pres have worked their way up the list.

about the melcor eqs, i see a pair on ebay now, just like mine but w/o the in/out switch, and knobs mising. the description mentions having an in/out switch but i don't see it on the control face.

i don't have a schemo, just a pinout. these have very a very good sounding bass-boost. in fact that's their strong point, IMHO. reminds me alot of flickinger. tight bass. so far i've only used the upper band for cutting freqs, with exception of boosting 3k on drum subgroup which sounds great! :grin:

 

[gar381]

Hi Peter

My AML-27 cards look just like the picture of MIKO's
earlyer in this thread. They are all orginal except for my
recaping and have orginal 1731 DOAs.

GARY

 

[gar381]

The way this card is set up the smaller the resistor is in the voltage divider the less neg feedback there is therefore more gain.

GARY

 

[clintrubber]

The way this card is set up the smaller the resistor is in the voltage divider the less neg feedback there is therefore more gain.

OK, so '3.1.2-style'. Hmm, do you have a scope and a squarewave-generator ? Would be interesting to see if 'the other instab.' was there as well - so for lowest gain (no resistor I assume then).

regards,

Peter

 

[gar381]

Here is a look at a 1kc sq.wave. on my tek. The waveform looks the same from min gain up to 55db and at all output levels up to clip.
Note input trafo pri seriesed and terminated at 600R. It seems to have a nice bit of a ring. Both of my cards have the same findings.

GARY

 

[gar381]

I think I'll run next week and see if I can find a ris.
around 600R that will handle a little power and term the output trafo and see if that will help the ringing.

GARY

 

[b3groover]

[quote author="Tungstengruvsten"]'Uncle Meat' is a Frank Zappa/Mothers of Invention Album from the late 60's....some killer tunes, lotsa musique concrete.

I have a handful of MAP AM27 docs including schematics and layouts for both the mic pre and the bridging line amp. I sold a bunch over the last 6 months to pay off racking up the 8 I kept. A friend and I racked up 2 as a prototype, they sound great, and we modded a pair of cards with Hardy 990's that sound stellar. Also there is a post on here somewhere from 'B3groover' on how he racked up a pair he purchased from me....if anyone needs the docs I can send em out![/quote]

Yes, this was my first "DIY" project. The thread is here:

http://www.groupdiy.com/index.php?topic=2805&start=0

I still haven't gotten pads on these yet. I plan on doing that this weekend and then trying them out on drums!!! I've used them on guitar, vocals, harmonica and... well, that's about it so far. They sound great! I'd love to put some Hardy 990s in there!!

Tungstengruven, you got any more?

 

[clintrubber]

Thanks for the pics gary. So you're saying the overshoot is identical (same percentage of signal-swing) for all gain-settings ? I'm not sure about what you say about the TXs, but I understand the ringing/overshoots will all be TX-related.

Bye,

Peter

 

[gar381]

Hi all

Here is the latest update on my AML-27 card experiments.

Found a 1watt 680R laying around the shop at work. I tried terminating the output trafo to see if it would reduce the ringing problem as shown
in my post above. ..NO BETTER the ringing was exactly the same. Time to play with the RC network across the secondary of the input trafo. The original is a 470p in series with a 2K7.. Using the method from Kev?s old Project 2 post:

http://recording.org/users/kev/Project2.htm


I replaced the 2K7 with a 20k pot I had laying around and feed the card a 10kc square wave. I played with the pot and it helped a lot. Then I replaced the 470p with a 1000p and I was able to flatten the top of the wave VERY nicely at about 11k8 on the pot. The 10kc wave was very
flat and had only a slight tilt to the leading edge rise time. The 1kc wave looked perfect!!

1000p with 11k8 in series..... PERFECT.

BIG Problem:
With this RC arrangement I can only get 40db of gain out of the card before it oscillates. I was getting 55db before oscillation with the old RC set up.

Just for fun I cranked the pot to 0R and wow 60db of gain was possible but my 10kc square wave now almost looks like a sine wave.

The more it rings the more stable the gain.
The nicer the wave form the less stable the gain.

I?ll play with it more next week when I get time.

GARY

 

[clintrubber]

Just for fun I cranked the pot to 0R and wow 60db of gain was possible but my 10kc square wave now almost looks like a sine wave.

I don't recall right away the openloopgain and/or gain-BW values Brad (bcarso) found in his simulations, but for that 60dB closed loop gain from this circuit you'll have not much (if any) gain-margin left at most frequencies. In other words, the open- & closed-loop curves start to become alike, so the circuit will approach a lowpassfilter with say ..dB gain up to a few hundred Hz and then start sloping downwards, filtering the 'squarewave-harmonics'.

regards,

Peter

 

[gar381]

here is a summary of known info, a rev. eng. schemo and my recaping notes:

Maybe this will help some folks. Corrections always welcome.

GARY

 

[jho-audio]

Well, I tried to power up a module after a full day of metal work on this past weekend. My result was an extremely quiet noise floor, very impressive, but, the problem is that there was no audio being passed through either.

Please, someone, help me to diagnose my connections:

1) out xlr pin 2
2) out xlr pin 3
3) out xlr pin 1
4) strapped to 7 with 330r
5) strapped to 8 with 610r - in xlr pin 3
6) strapped to 7
7) strapped to 6
8) strapped to 5 with 610r - in xlr pin 2
9) n/c
10) strapped to 4 with 330r
11) n/c
12) n/c
13) -15v
14) chassis (tried direct to 0v as well)
15) +15v

remaining:
- in xlr pin 1 connected to chassis
- power 0v connected to chassis (tried direct to pin 14 as well)

Anything I am missing? I am away from my bench until this coming weekend. Should I be grounding differently? Cap replacement? I don't think that this is related to the AML-27 because I tried four different modules. Guessing that this is a connection fault or pour grounding issue.

Please help. Thanks.

 

[MikoKensington]

Digging up the bodies. So I successfully racked my MAP cards. Now I have the Melcor brand cards in a bin. I was wondering what effect installing a Cinemag 1:7 input transformer would have. The Cinemags are big heavy and begging to be put to use. Any input? (no pun intended)

 

[jho-audio]

Well, I am finally finished with the aml-27s :grin:

These cards sound great! I added a D.I. to each channel and I am extremely impressed with how good it sounds.

Miko, unless you need excess gain I dont think you will need to add the cinemag transformers. The aml-27s have enough gain for my ribbon mics (beyer m160's and m500's) with headroom to spare on a quiet acoustic guitar.

Thanks so much to y'all for the help and info. Now its time for me to finish up my 1108's and AM-16's :grin:

Photos:

 

[MikoKensington]

Congrats! You win! Looks good. I guess I should get going, huh? Did you only re-cap one of the cards?

 

[jho-audio]

Thanks, yeah, only one re-cap so far, that is my next task. You will not be disappointed with your time spent on these cards. :thumb:

 

[gar381]

Hi jho

VERY BEAUTIFUL JOB!!

Between you and Miko?s MAP/Melcor EQ project, I?m
VERY inspired!! Now I MUST finish my AML-27
Project!!

Did you use the AM-27 resistor gain table for your
Gain switch? How did you rig your DI inputs?
It looks like you used relays for phase and pad.

I AM IMPRESSED!!!


:thumb: :thumb: :thumb: :guinness: :sam: :guinness: :sam:

GARY

 

[jho-audio]

Thanks Gary!

The relay board for pad, polarity and phantom (pass through, non relay) came from Peter Purpose in this thread: http://www.groupdiy.com/index.php?topic=7668&highlight=relay+pad

I used the AM27 gain table as-is up to 55db, works perfectly.

For the DI input, un-solder the green wire from the input transformer, add a switched 1/4" jack from the green wire to the jack and then back to the same spot you un-soldered the green wire from, one more cable which goes to ground (I used the bolt on the bottom of the input transformer for ground). For clarity I have added to your image the connections I made.

 

[Bo Hansen]

jho-audio,

Some small hints about your AML-27 DI-modification.

If the signal source not have any ground referense on the output (a resistor to ground after the output capacitor) you shall have a resistor on the 1731 op-amp +input to ground, approx. 1 Mohm is ok for a DI-input.

Do the Di-jack insert after the R1, C1 transformer compensation, so this not influence the Hi-Z signal sourse you connect to the DI-input. (a passive guitar/bass)

To secure the 1731 op-amp life, connect a 0,1 uF capacitor (ok for 1 Mohm input resistor) in serial with the input jack, to protect for dc-voltage from a outside signal source.

You can also connect a zenerdiode protection and a "clamp-diode" protection to get both "braces and waistbelt"
It is pity to destroy a old nice orginal Melcor 1731 op-amp, if you happen to connect some strange thing to the DI-input.

Take a look on my API 312 DI-modifiaction
http://web.telia.com/~u31617586/ that is very similar.

--Bo

 

[jho-audio]

Thanks for the great advice Bo, I will be sure to fix this up along with a re-cap this week.

Cheers, Jason