modifying Yamaha MLA7 towards Trans-Amp-like circuit

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JR.
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Sat Jan 12, 2019 1:04 pm

AMRinput.pdf
(409.67 KiB) Downloaded 35 times
This is input strip from AMR console... I basically used existing in-house Peavey two opamp topology.

I have no idea who did this first, not me.

JR

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by emrr » Sat Jan 12, 2019 2:19 pm

Thanks guys, will let this soak in a bit.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by mediatechnology » Sat Jan 12, 2019 3:12 pm

Image
Peavy AMR Input Module Mic Preamp.

Thanks JR.
2SD786s noted.
I see that Joe Neil and I aren't the only ones who liked the idea of inductors in the gain leg.

Basically speaking:
U1B in the AMR is IC101 2/2 in the MLA7.
IC101 1/2 is "repurposed" to become a unity gain inverter to play the role of U1A.
Resistors are then connected from the outputs to the input transistor emitters.
There are some other resistors to remove/change and some compensation to be modified but not a lot of hacking.

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Sat Jan 12, 2019 3:56 pm

Yup the 2sd786 (npn) was already in the Peavey MRPS system so I didn't bother to bring in a new part number just to use my favorite 2sb737 (pnp) even though lower noise on paper, not worth the effort.

Both obsolete now...

JR

[edit and no I still haven't found my bag of 737s :lol: /edit]

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by mediatechnology » Sat Jan 12, 2019 6:42 pm

JR. wrote:
Sat Jan 12, 2019 3:56 pm
[edit and no I still haven't found my bag of 737s :lol: /edit]
I was hoping as you were digging the drainage trench you'd find where you'd buried them in the backyard.
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Sat Jan 12, 2019 8:36 pm

mediatechnology wrote:
Sat Jan 12, 2019 6:42 pm
JR. wrote:
Sat Jan 12, 2019 3:56 pm
[edit and no I still haven't found my bag of 737s :lol: /edit]
I was hoping as you were digging the drainage trench you'd find where you'd buried them in the backyard.
I did find the pair of reading glasses that dropped in the trench, a week later...

I bet they are there, just under/behind something :lol:

Probably won't ever be "found" but discarded as trash with the rest of it... :oops:

JR

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by emrr » Tue Jan 15, 2019 11:34 am

hahahahahahahahahahahahaaaaaaaa!!

Wow. Dear JW/AU.....

Got it, mostly....partly....

The Self book; somehow that fell off into a black hole in a blind spot, I've never seen it. Very clear descriptions.

I found the documents for the next gen MLA8, and it incorporates those changes, moves to 4580 op amps, adds a HPF section followed by two op amp stages pushing balanced output. Almost cheap enough on used market to ignore DIY; but I have two of the MLA7 already I'd like to hack on.

Control of DC output via the servo in figure 13-3: WHERE does that particularly matter? The inverter? Effect on THD?

I suppose that also may relate to choices made around the op amp selection too? Figure 3 of the Valley TA-103 document (better HERE) mentions using 5532/5534 for driving lower Z lines, though I like the sound of TLE207x family fine and it appears to be fine for a direct sub in the Yamaha. Is it unreasonable to use the inverter output as the rest of a balanced output? I assume the standard approach is to feed a differential amp with that pair of signals. I was previously thinking about cutting output connector traces to add impedance balancing to the outputs.

The lower gain 'padless amplifier' without the servo in Self figures 13.5-13.6, I'm taking it that's a dual section pot used to control input and output stages simultaneously.

Sidebar: the comments about the 4 resistor pad in 13-4 are interesting, in that I feel I rarely see that used. Probably because I'm usually looking at transformer inputs where the 3 resistor does the job, yet it's a 3 in the Yamaha; easily changed.

It's a bit astonishing to remember the MLA7 was the accessory input amp for the first EXPENSIVE Yamaha digital mixing system(s) in the late '80's, you'd think it'd been a bit more premium. I always used them at much lower gains in my rock club remote recording era, many times with the pads on.
Best,

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by mediatechnology » Tue Jan 15, 2019 1:04 pm

Control of DC output via the servo in figure 13-3: WHERE does that particularly matter? The inverter? Effect on THD?
I don't think you'll need the servo since Rg is AC-coupled.
I suppose that also may relate to choices made around the op amp selection too? Figure 3 of the Valley TA-103 document (better HERE) mentions using 5532/5534 for driving lower Z lines, though I like the sound of TLE207x family fine and it appears to be fine for a direct sub in the Yamaha.
Whatever works for you and can be made not to oscillate. I have some TLE207X somewhere. Can it drive low impedance loads or does it have a TL072 light-weight output?
Is it unreasonable to use the inverter output as the rest of a balanced output? I assume the standard approach is to feed a differential amp with that pair of signals. I was previously thinking about cutting output connector traces to add impedance balancing to the outputs.
Yes the inverter can be used to provide the anti-polarity output. In the two op amp topology of Self and Roberts common mode rejection is realized in the differential stage connected to the input transistor collectors. There's no need for a third differential amp to obtain common mode rejection like there is in the TransAmp, "Cohen," THAT151X, INA217 etc. (My hunch is that the two op amp lacks the symmetry of the three op amp which might affect HF CMRR but I'm not sure its worth it in most cases. The improvement you're making bringing NFB to the emitters is significant.)
The lower gain 'padless amplifier' without the servo in Self figures 13.5-13.6, I'm taking it that's a dual section pot used to control input and output stages simultaneously.
Yes I believe so. Need to look at it to see if we can call it "shared gain." (Had a look-see. No, it's not. It's a Ted Fletcher "SuperBal" gain stage.)
Sidebar: the comments about the 4 resistor pad in 13-4 are interesting, in that I feel I rarely see that used. Probably because I'm usually looking at transformer inputs where the 3 resistor does the job, yet it's a 3 in the Yamaha; easily changed.
I don't grok his pad man. IIRC its also on the dry side. I've always used a three R pad on the wet side.
It's a bit astonishing to remember the MLA7 was the accessory input amp for the first EXPENSIVE Yamaha digital mixing system(s) in the late '80's, you'd think it'd been a bit more premium. I always used them at much lower gains in my rock club remote recording era, many times with the pads on.
https://ka-electronics.com
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Tue Jan 15, 2019 1:06 pm

emrr wrote:
Tue Jan 15, 2019 11:34 am
hahahahahahahahahahahahaaaaaaaa!!

Wow. Dear JW/AU.....

Got it, mostly....partly....

The Self book; somehow that fell off into a black hole in a blind spot, I've never seen it. Very clear descriptions.
I don't have to worry about losing my copy .... never bought one. :lol:
I found the documents for the next gen MLA8, and it incorporates those changes, moves to 4580 op amps, adds a HPF section followed by two op amp stages pushing balanced output. Almost cheap enough on used market to ignore DIY; but I have two of the MLA7 already I'd like to hack on.

Control of DC output via the servo in figure 13-3: WHERE does that particularly matter? The inverter? Effect on THD?
I went back and looked at the design with servo and meh... for one thing the TL07x is not noted for DC performance and secondly it is servoing the primary opamp output to 0V in an ideal world that might balance the currents of the input pair if the 4.7k to -17V are precision, and op amp dc input offset is low, etc. In theory if he servos the opamp output to be 0V why use an output DC blocking capacitor?

If I was going to invest the extra silicon to add a servo to that circuit I would servo to make the two input device emitters the same voltage so you can lose the 470uF cap in series with the gain pot. But that has issues too... Wayne has far more experience with that (I think).

Looks to me like he had a section of op amp left over... lets throw a servo in, that will impress the unwashed. :lol: In console design we generally don't throw extra stuff in times 20 or 30 identical channel strips, unless it is useful.
I suppose that also may relate to choices made around the op amp selection too? Figure 3 of the Valley TA-103 document (better HERE) mentions using 5532/5534 for driving lower Z lines, though I like the sound of TLE207x family fine and it appears to be fine for a direct sub in the Yamaha. Is it unreasonable to use the inverter output as the rest of a balanced output? I assume the standard approach is to feed a differential amp with that pair of signals. I was previously thinking about cutting output connector traces to add impedance balancing to the outputs.
keep in mind that application notes written decades ago had a smaller palate of op amp choices. Back in the day I used truckloads of TL07x and 553x, because they were very good for the money, for the time.

JR
The lower gain 'padless amplifier' without the servo in Self figures 13.5-13.6, I'm taking it that's a dual section pot used to control input and output stages simultaneously.

Sidebar: the comments about the 4 resistor pad in 13-4 are interesting, in that I feel I rarely see that used. Probably because I'm usually looking at transformer inputs where the 3 resistor does the job, yet it's a 3 in the Yamaha; easily changed.

It's a bit astonishing to remember the MLA7 was the accessory input amp for the first EXPENSIVE Yamaha digital mixing system(s) in the late '80's, you'd think it'd been a bit more premium. I always used them at much lower gains in my rock club remote recording era, many times with the pads on.

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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by mediatechnology » Tue Jan 15, 2019 1:10 pm

Back in the day I used truckloads of TL07x and 553x, because they were very good for the money, for the time.
WRT to the 5532 they still are a good value: https://proaudiodesignforum.com/forum/p ... f=6&t=1089
https://ka-electronics.com
Learn to code.
"Mr. Sociopath Goes to Washington" https://www.zerohedge.com/news/2019-02- ... washington
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"Knowing Keeps Us Free" https://www.youtube.com/watch?v=5lC3mpG60sw
"Nick Sandman: The Truth in 15 minutes" https://youtu.be/lSkpPaiUF8s

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