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Yamaha TG77 balanced output (and other mods)
Yamaha TG77 balanced output (and other mods)
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Old 27th March 2018, 04:56 AM   #1
minimaliszt is offline minimaliszt  United States
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Join Date: Jan 2017
Default Yamaha TG77 balanced output (and other mods)

Hey diy folks,

Forgive any misunderstandings in the following. I'm a musician, and I like to maintain my instruments, but I'm not an EE, so I like asking for help...

I got my hands on this old TG77 for some upcoming performances (it's awesome: my wife likes to joke it sounds like Stranger Things, which is absolutely on point), but I've been thinking I might make a few practical modifications, and then eventually a major analog overhaul (it is fairly noisy).

First and foremost, all the outputs are unbalanced and have a relatively high output impedance (about 10kohms). Generally folks just use DIs for these old synths, which work fine, but it would be nice to avoid the extra equipment/cabling/noise. Seems like the perfect setting for an implementation of THAT1646, but the Yamaha schematic has raised a few questions, which hopefully I can get some help on here.

Attached is the schematic of the main output section, starting from the DAC and continuing through to the line drivers.

The first big question for me is where to tap into the signal. At this point, I'm inclined to tap into the same points where signal is being diverted to the headphone amp (right after IC 303/307) and similarly output a sort of "master mix," drilling a couple extra holes in the back panel and adding two more TRS jacks. The main outputs (1/1+2 and 2) look like a maze of interconnections, especially JK302, which appears to sum channels 1 and 2 together, so it seems wise to avoid messing with all of that, but I'm open to other suggestions.

The other consideration is that high output impedance. ICs 301/3/6/7 are all RC4558, which has an output current of 10mA. I'm not totally sure, but it looks like the transistors at the outputs (Q301/2/3/4) are acting as something of a line driver. If I changed these amps for something more modern, like OPA1662/1692/etc (something I'm anticipating doing anyway), would I then remove these transistors, or am I misunderstanding something about their purpose here?

In terms of other modifications, the DAC looks like it could use a little more decoupling, and all the chips are pretty outdated––IC 305/309 are M5238P, which could be changed to OPA1642, and the rest are RC4558, as I already mentioned. The bipolar caps are all Panasonic CE, but they're all probably 30 years old, so they could all get a swap.

Any and all suggestions are welcome!
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Old 27th March 2018, 12:13 PM   #2
Enzo is online now Enzo  United States
diyAudio Member
Join Date: Sep 2004
Location: Lansing, Michigan
You are totally misunderstanding the transistors. Q301-304 are mute transistors. Look at the schematic. If they turn on, they short across the signal path, silencing the output. If you remove them, you may have noises at power up, etc.

Where to tap? WHy tap at all, use the existing output. Take the signal across C301, C307, right at the output. That will retain all mixing if running mono. From there, you can add an electronic driver circuit, or just wire in a pair of signal transformers.

But yes, you could use L1 and R1 from IC303.

These also work, and require no modification to the unit.
Audix T-50K Inline Impedance Matching Transformer | Musician's Friend
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Old 27th March 2018, 02:44 PM   #3
minimaliszt is offline minimaliszt  United States
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Join Date: Jan 2017
That totally makes sense... thanks!
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Old 27th March 2018, 10:53 PM   #4
PRR is online now PRR  United States
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Join Date: Jun 2003
Location: Maine USA
Yamaha TG77 balanced output (and other mods)
Originally Posted by minimaliszt View Post
...outputs are unbalanced and have a relatively high output impedance (about 10kohms)....
Output is 1K for all practical purpose. (Add 1 Ohm inside the opamp, the 10uFd, shunt with the 10K and the OFF mute transistor... 10K f.a.p.p.)

A 1K source can drive 300 feet of 30pF/ft cable with small loss at 20KHz.

You could tack another 1K across the 1K series resistors for a 500r output, slightly more robust.

Balanced source is not so important as balanced (differential) receiver. Since most board inputs today do this, I would not be chopping holes in a vintage '77.

I am mildly startled that Yama used the '4558 opamps at this late date. However a "hotter" chip may have stability troubles in a '4558 layout. If this really became the core of a good-paying band, I'd think about a side-car to take the modest outputs and bump-up level and drive.

I'd take this all very cautiously, with much-much ear-testing at every small step. You do not want to lose the 1990s sound.
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Old 27th March 2018, 11:16 PM   #5
MAAC0 is offline MAAC0  Portugal
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Join Date: May 2010
Try a pseudo balanced output. A 1K 10K combination like on the tip applied to the sleeve connected to ground.
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