modifying Yamaha MLA7 towards Trans-Amp-like circuit

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

Post by emrr » Tue Jan 15, 2019 2:20 pm

mediatechnology wrote:
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.
JR, your POV makes sense to me on the servo.
Servo and non-servo both look AC coupled. I'm failing to see the real point of it being there with all the AC coupling. Should I take it that the inverter path compounds DC offset of the U1A path, thus the servo?
mediatechnology wrote:
Tue Jan 15, 2019 1:04 pm
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?
Looks like the same light-weight output, just higher slew and bandwidth. But like the existing MLA7, just a 4558 driving the output and spec mentions 10K loads, I wouldn't be doing worse.
mediatechnology wrote:
Tue Jan 15, 2019 1:04 pm
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.)
That was the starting thought!
mediatechnology wrote:
Tue Jan 15, 2019 1:04 pm
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.)
Looks like Self fig 14.11 to me, same thing I think. I'm still unclear on the gain pot here, looks like 2 that must be on the same shaft. Can't imagine two gain controls on any of the consoles he's referencing.
mediatechnology wrote:
Tue Jan 15, 2019 1:04 pm
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.
I've got a pad built into this thing already. I guess I can experiment with inducing common mode noise and compare real world. All my historical usage was LOUD, these days trending much quieter. I'll have to see what sort of gain range it falls into at first hack....autodidact and all.....
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Doug Williams
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JR.
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Tue Jan 15, 2019 3:01 pm

mediatechnology wrote:
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
They were respectable performers even by today's standards when used conservatively. It is hard to imagine that we've had then to use since the 70s....

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

Post by mediatechnology » Tue Jan 15, 2019 3:04 pm

The gain controls of the "padless" are on a common shaft.

I didn't extract the figure for 13.3 in the pdf but Self states in the text that the front-end gain of 13.3 is higher than 13.5.
IIRC Rfb is 82K vs 2K2.

He writes:
The lower feedback resistors mean that no servo is required to correct the DC conditions.
What I don't see is DC gain anywhere.
I'm going along with the theory that there was an unused op amp or its correcting the large inherent offsets of the TL072.
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by emrr » Wed Jan 16, 2019 11:26 am

Curiosity veer; I looked back at what Mackie was doing in the 8 bus series. I recall times I liked the Yamaha sound over the Mackie by a bit, it was probably the distortion on the loud rock bands I was tracking at the time.
Mackie 8 bus SCH_preamp.jpg
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mediatechnology
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by mediatechnology » Wed Jan 16, 2019 6:31 pm

emrr wrote:
Wed Jan 16, 2019 11:26 am
Curiosity veer; I looked back at what Mackie was doing in the 8 bus series. I recall times I liked the Yamaha sound over the Mackie by a bit, it was probably the distortion on the loud rock bands I was tracking at the time.

download/file.php?id=554&mode=view
C135 and C136 look odd to me. Don't usually see them there.
If it were me I would have at least put C135 at the emitter of Q104 to maintain symmetry.
It looks like it has a built-in HF common mode imbalance.

The input transistors also run open loop like the MLA7 but they have higher gain due to the connection of the transistors.
Forget the name of that Darlington configuration...
(Had to look up the PNP/NPN complimentary Darlington: Sziklai pair. Named after Hungarian George Sziklai.)

I have lots of new stock JRC 4560s if you like the sound of the Mackie.
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Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit

Post by JR. » Thu Jan 17, 2019 2:21 pm

mediatechnology wrote:
Wed Jan 16, 2019 6:31 pm
emrr wrote:
Wed Jan 16, 2019 11:26 am
Curiosity veer; I looked back at what Mackie was doing in the 8 bus series. I recall times I liked the Yamaha sound over the Mackie by a bit, it was probably the distortion on the loud rock bands I was tracking at the time.

download/file.php?id=554&mode=view
C135 and C136 look odd to me. Don't usually see them there.
If it were me I would have at least put C135 at the emitter of Q104 to maintain symmetry.
It looks like it has a built-in HF common mode imbalance.
+1 asking for trouble... back a hundred years ago I used a driven cable shield to reduce effective capacitance at those very nodes for a gain pot located too far away for grounded cable shields.

FWIW Mackie had a bad reputation for poor RF immunity back in their early days... maybe that was the cause or their half__ attempt to manage that. I notice inductors (beads?) in series with inputs.

JR
The input transistors also run open loop like the MLA7 but they have higher gain due to the connection of the transistors.
Forget the name of that Darlington configuration...
(Had to look up the PNP/NPN complimentary Darlington: Sziklai pair. Named after Hungarian George Sziklai.)

I have lots of new stock JRC 4560s if you like the sound of the Mackie.
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by emrr » Sat Jan 19, 2019 10:50 am

mediatechnology wrote:
Wed Jan 16, 2019 6:31 pm
I have lots of new stock JRC 4560s if you like the sound of the Mackie.
Oh no, I put masking tape over the XLR inputs and started using other preamps 2 decades ago. I only used the tape input which bypasses all of that. Veer because I listened to this side by side with the MLA7 at the time.

mediatechnology wrote:
Wed Jan 16, 2019 6:31 pm
C135 and C136 look odd to me. Don't usually see them there.
If it were me I would have at least put C135 at the emitter of Q104 to maintain symmetry.
It looks like it has a built-in HF common mode imbalance.
I thought that very odd too.
Best,

Doug Williams
Electromagnetic Radiation Recorders

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