Re: modifying Yamaha MLA7 towards Trans-Amp-like circuit
Posted: Sat Jan 12, 2019 12:04 pm
I have no idea who did this first, not me.
JR
https://www.proaudiodesignforum.com/forum/php/
https://www.proaudiodesignforum.com/forum/php/viewtopic.php?t=1088
I did find the pair of reading glasses that dropped in the trench, a week later...mediatechnology wrote: ↑Sat Jan 12, 2019 5:42 pmI was hoping as you were digging the drainage trench you'd find where you'd buried them in the backyard.
I don't think you'll need the servo since Rg is AC-coupled.Control of DC output via the servo in figure 13-3: WHERE does that particularly matter? The inverter? Effect on THD?
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?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.
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.)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 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.)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.
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.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.
I don't have to worry about losing my copy .... never bought one.
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?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?
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.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.
WRT to the 5532 they still are a good value: https://proaudiodesignforum.com/forum/p ... f=6&t=1089Back in the day I used truckloads of TL07x and 553x, because they were very good for the money, for the time.