YAMAHA A-S701 - tear-down, thoughts about the internals & few measurements

[Head_Unit]

That’s new. Just was wondering at 4ohms will the marked power diff from the 701/801 will give better results. Really like the look of the amp and the measurements show that their engineers really do good work..

Yamaha A-S801:100 watts continuous 8 ohms, 290 dynamic into 2 ohms
A-S500: 105, 220.
I'd call that a tie at least to some insignificant approximation. To me, to have significantly more power you need to like triple the available output, since our hearing is logarithmic.

 

[dgk]

I've read this excellent thread with a great deal of interest for a couple of reasons:

1/ I have a A-S700 that seems to have a +6db higher output on the right channel. Same on all pre-amp input combinations so I'm sure the issue is in the power amp section. I've looked at the technical manual / schematics and can't find any obvious (to me) way to adjust the bias apart from using the front panel balance control (which means I can't use any of the direct switching features). Any insight from you guys familiar with this series of amplifiers would be greatly appreciated.

2/ What's the real technical problem with dropping speaker output down via a 470 resistor to drive headphones? Using HD-650's I have done reference comparisons between the S700 headphone output and the headphone amp in the Marantz CD-6006 after ensuring exactly the same channel volumes (using a -5dbFS 1kHz sine wave file). I can't hear any difference at all....

 

[dgk]

......seems to have a +6db higher output.... should say +0.6db!

 

[Cahudson42]

Though I am unsure if final stage bias is the problem, many of these old Yamahas have an extremely crude bias adjustment done by either opening (or 'clipping open') a particular vertically mounted resistor. Or reclosing it, if previously clipped open.

Read the service manual carefully.. Some appear have to deleted details on this.

 

[dgk]

Though I am unsure if final stage bias is the problem, many of these old Yamahas have an extremely crude bias adjustment done by either opening (or 'clipping open') a particular vertically mounted resistor. Or reclosing it, if previously clipped open.

Read the service manual carefully.. Some appear have to deleted details on this.

I did see a procedure like that related to idling current. Does that make sense?

 

[Cahudson42]

I did see a procedure like that related to idling current. Does that make sense

Yes. Idling current is bias adjustment. While I do not suspect this may be your channel difference problem, the procedure mentions checking with a multimeter between two test points. Perhaps if there is a substantial difference between channels, it is the problem.

You can refine the adjustment by replacing the single resistor with a variable-series pair. Do Not replace it with just a variable - misadjustment could give an unpleasant fireworks surprise if you do. I forget the value of the resistor - but say it is 1k. I'd replace with a 500 ohm fixed in series with a 1k variable, for a 500 to 1.5k range. Set the variable midway - 1k total - to start.

You should also be checking for DC offset before connecting any speakers after doing this.

I invite any of our ASR hardware experts to correct, modify, or add to this - before you attempt it.

 

[restorer-john]

Though I am unsure if final stage bias is the problem, many of these old Yamahas have an extremely crude bias adjustment done by either opening (or 'clipping open') a particular vertically mounted resistor.

I'd hardly call it "old". The AS-700 and 710 (current) are essentially the same, one has a D/A sub board. Nothing else has really changed in 12 years.

0.6dB difference is unlikely to be anything to do with the power amplifier stage or bias. Increasing or decreasing bias doe not change the basic gain of a power amp stage.

@dgk You would need to ascertain where the gain error is creeping in. It could be all the way back at the electronic input selector, the digitally controlled volume or one of the gain/buffer stages prior to the power stage. It could also be the power amp gain itself. What test gear and knowledge level do you have?

 

[dgk]

@dgk You would need to ascertain where the gain error is creeping in. It could be all the way back at the electronic input selector, the digitally controlled volume or one of the gain/buffer stages prior to the power stage. It could also be the power amp gain itself. What test gear and knowledge level do you have?[/QUOTE]

I did take a look at the block diagram and tried a few inputs other than the CD input and noted exactly the same difference. Also, and probably more telling, with CD DIRECT IN activated exactly the same difference is noted. So wouldn't that implicate the final stage? I took a quick look at the detailed schematics and they seem consistent with the relevant area in the block diagram but I might have missed something.

With guidance I'm comfortable poking around in these things and their schematics and have some basic test gear like Fluke multimeter, cheap usb oscilloscope but nothing fancy.

 

[dgk]

@dgk .... the digitally controlled volume ...

Just realized CD DIRECT does not bypass this so maybe that's where I should start?

 

[restorer-john]

Just realized CD DIRECT does not bypass this so maybe that's where I should start?

I'd have to have a look at the schematic later, but the 300/301/500/501 and 700/701 are all extremely similar apart from a CD direct buffer stage and IIRC another buffer stage to account for the losses in the digital vol/source selector IC.

The easiest was to determine where the problem is, is to start immediately after the source selection, where the signal is sent out of the main IC and then returns, gets looped into the dig-attenuator and then sent back out again. Look at the waveforms for each channel when fed a mono signal. They should be identical or very close. Then progressively go through the stages, comparing amplitudes for each channel.

Or, simply combine the L+R signals right at the power amp input (there's a L/G/R connector on the main amp PCB near the front) and check to see if it's balanced at the speaker terminals. That will rule out the power amp stage balance in one go.

Bear in mind, the volume pot is not traditional at all. Yes it's a two gang pot, but only one wiper/track is used to generate a voltage read by the A/D in the micro, which in turn feeds the digitally controlled volume IC. So you can't probe the volume pot terminals like a normal old school amp.

Put it this way, the implementation Yamaha uses provides perfect channel balance and particularly L/R tracking over the full range, so a mis-balanced unit means there is a problem.

 

[audio_tony]

2/ What's the real technical problem with dropping speaker output down via a 470 resistor to drive headphones? Using HD-650's I have done reference comparisons between the S700 headphone output and the headphone amp in the Marantz CD-6006 after ensuring exactly the same channel volumes (using a -5dbFS 1kHz sine wave file). I can't hear any difference at all....

There are a couple of (technical) issues here (even though you can't hear any difference).

Having the 470 ohm in series with the output of the amp to drive headphones means that the output impedance of the amp is raised significantly. This will affect the damping factor, and will also result in differing output levels with different headphones (for a given volume setting).

Additionally, with some amps, the bias may be less than optimal at very low output levels, and such low levels are usually required for headphone listening, so in some cases crossover distortion may be quite evident at lower volumes, but will disappear as the volume increases.

I suspect with modern* amps, the bias issue is probably non existent.

*probably anything designed post 1975!!

 

[trl]

2/ What's the real technical problem with dropping speaker output down via a 470 resistor to drive headphones? Using HD-650's I have done reference comparisons between the S700 headphone output and the headphone amp in the Marantz CD-6006 after ensuring exactly the same channel volumes (using a -5dbFS 1kHz sine wave file). I can't hear any difference at all....

Not quite an etalon the CD-6006, but definitely better than couple of 470 Ohms resistors:

​

You should try some sensitive 16-32 Ohms headphones (IEMs, if possible), then some planars, then see if any differences appear.

 

[dgk]

Put it this way, the implementation Yamaha uses provides perfect channel balance and particularly L/R tracking over the full range, so a mis-balanced unit means there is a problem.

Thanks for all this guidance. Its given me some confidence to go ahead and troubleshoot this puppy. I'll let you know what I find!

 

[dgk]

You should try some sensitive 16-32 Ohms headphones (IEMs, if possible), then some planars, then see if any differences appear.

Thanks for the CD6006 schematic, I have not been able to find that to date and was curious how they had implemented things. Regarding going down the suggested path with other headphones, with my 57yo golden tin eutectic ears I can't really justify the expense and time.

What would be nice to know is the best way to match the S700 speaker output with my HD650's? Difference resistors or some kind of buffer?

 

[audio_tony]

What would be nice to know is the best way to match the S700 speaker output with my HD650's? Difference resistors or some kind of buffer?

Simple solution; Buy a headphone amplifier (plenty of recommendations here on ASR) and plug it into the "REC" outputs on the amp - and when you listen with headphones, simply turn off the speakers.

That'll give you the best possible headphone experience.

 

[dgk]

Simple solution; Buy a headphone amplifier (plenty of recommendations here on ASR) and plug it into the "REC" outputs on the amp - and when you listen with headphones, simply turn off the speakers.

That'll give you the best possible headphone experience.

That would be simple certainly, but my question is really a technical one. If Yamaha were do do this properly, what should they have done? I recalled trl mentioning their more expensive amps having dedicated headphone amplification. I was able to find the service manual for the S2100 and, yes indeed, Yamaha don't use the final stage speaker output at all. Does this mean there is no right way to properly implement something between the speaker output and headphones?

 

[audio_tony]

Does this mean there is no right way to properly implement something between the speaker output and headphones?

The correct way to do it, would be to implement a dedicated headphone amplifier right after the preamp / volume control stage, before the main power amp stage.

A lot of integrated amps don't even have a dedicated preamp stage, they just run the power amp at high enough gain that a preamp stage is not needed.

In that instance - they could still have placed a dedicated headphone amp stage directly after the volume control.

 

[restorer-john]

You should try some sensitive 16-32 Ohms headphones (IEMs, if possible), then some planars, then see if any differences appear.

Why? There's little point other than a practical demonstration of Ohm's law. He has a pair of HD-650s. They are a nominal 300R H/P and with the source impedance of the amplifier being 470R, he will have more V swing available than any little toy headphone amplifier out there. The 650s also have a pretty benign impedance curve (300Rmin 500R max), so FR errors will be inaudible.

Personally, I would fix the amplifier to be perfectly balanced and use the headphone socket on it and enjoy. The benefits are massively increased available voltage swing comared to a dedicated H/P amp and of most importance, protection. DC protection is built in to the amp and will protect the headphones in a fault situation. Most standalone H/P amps have none.

 

[AnalogSteph]

The 650s also have a pretty benign impedance curve (300Rmin 500R max), so FR errors will be inaudible.

We're talking 2.4 dB peak-peak. I've been able to plainly hear less than that. These are relatively broadband colorations after all. If we want a "not inaudible but probably OK for most people" level of 1 dB, these cans should see no more than about 110 ohms, and for a stricter 0.5 dB requirement it should be sub-50 ohms. (I've got a little calculator for this.)

 

[restorer-john]

We're talking 2.4 dB peak-peak. I've been able to plainly hear less than that. These are relatively broadband colorations after all. If we want a "not inaudible but probably OK for most people" level of 1 dB, these cans should see no more than about 110 ohms, and for a stricter 0.5 dB requirement it should be sub-50 ohms. (I've got a little calculator for this.)

Cool calculator.

Here's a cute little graphic from HeadFi:

As I said, the HD-650 is a pretty benign load, so the FR deviations are hardly bothersome. With a 470R source impedance he'll get a barely noticeable rise in the lowest octave only.

Again, don't ignore protection. I've spent many decades repairing amplifiers and speakers from failed electronics. Headphones are extremely fragile and without DC protection and overcurrent limiting (series resistors) they vaporize in a split second. The Yamaha has onboard protection for DC, overcurrent and the series resistors prevent damage. Something I would consider as absolutely essential with a US$500 pair of cans wouldn't you?