Onkyo A7 question

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I am refurbishing an Onkyo A7 and in the circuit for each of the four output transistors there are 3.3uf 63v electrolytic capacitors bypassed by 0.001(?) ceramics that connect between VCC (+ and -) and the collectors of the output transistors.

I am struggling to understand the purpose of these

I have attached the circuit

Thanks
 

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thanks Cliff, but what is their purpose?

Decoupling. Google it.

Supply fast current pulses into the output transistors (and then the speaker!) which may not be available from the main PSU reservoir cap which may be a long way away through (inductive) wire.

Local decoupling is "A GOOD THING", and never does any harm. The higher the working frequencies the more important it becomes.
 
"You would be VERY hard pressed to measure any difference, let alone hear it. "

Not so.

The value of this electrolytic will change the character of the bass more than you know.

I suggest using as big as will fit the board.

I think you will find values in the range of 10µF~22µF best.

If you go 100µF (or larger) you will need a bigger secondary bypass cap (as mentioned in post #9).

These caps work hard, and frequently fail. On one brand of amplifier I serviced you would find the cans of these caps rattling around inside the chassis, the ripple current these caps provide is quite large. Another brand this part never failed, they used a 10µF film cap in this location (too big to retrofit for most amps).
 
Why are you changing them??

Are they faulty, or are you infected with the audiophool's "must swap caps" bug?

They are either alu or tantalum polarized electrolytics.

I am not the person to empathise about cap rolling.

Good luck. Do read stuff. Lots of it.

I've replaced thousands of caps over the years in broadcast gear. Not _all_ were bad but if there are several all of the same value and manufacture and they ARE bad, I will definitely replace all of that value. Some of those caps may only be a few years old. Keep in mind most of the units are on 24/7 and heat is the biggest culprit.

 
My pdf crashed when I tried to zoom in (my eyesight is terrible and deteriorating), but "in general" smaller electrolytics have higher resonance than larger values. I say "in general" because electrolytics vary a lot in performance parameters. A specific 4.7 uF or even 10 uF capacitor might have a higher resonance than a "general purpose" 3.3 uF capacitor. You didn't mention which Nichicon series capacitors you have but they make a whole bunch of different capacitors for a reason.

Miniature Type Aluminum Electrolytic Capacitors

I look at what's available and try to buy at least 10 so I get a discount. For local decoupling I use "audio grade" capacitors ;) which ostensibly are supposed to give lower distortion. These capacitors are a waste if they aren't very close to the active device (op amp, transistor, etc), and other types of electrolytic capacitors provide superior ripple current etc for power supply and reservoir caps. Of course the bypass network must include a very high resonance capacitor ( ceramic is ideal; resonance matters more than value).

In my headphone amp, I used 10 uF Nichicon KZ in parallel with another pair of Nichicon surplus caps (power supply types) in parallel right next to each other, and right next to each 49960 buffer chip, along with a 0.1 uF ceramic cap on the other side of the board, right on the chip power supply pins. I used 10 uF KZ for the op amp local bypass too (along with the 0.1 uF cap).

Cliff notes - if your 4.7 uf Caps have a high enough resonance, it will be equal or superior to the original caps for this application.

As djk noted, these caps work harder than you think. A 10 uF capacitor with equal or higher resonance would be an upgrade. Higher temp and extra long life (2000+ hours) is just a bonus.

I have not measured the difference in performance of my circuits with different electrolytics too much. I just try to put the right cap in the right place, according to its parameters. I do buy more of the same type if I like them.

Look at the datasheets. They answer most or all of your questions.
 
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"You would be VERY hard pressed to measure any difference, let alone hear it. "

Not so.

The value of this electrolytic will change the character of the bass more than you know.

I suggest using as big as will fit the board.

I think you will find values in the range of 10µF~22µF best.

If you go 100µF (or larger) you will need a bigger secondary bypass cap (as mentioned in post #9).

More than I know? :rolleyes::rolleyes:

The main reservoirs are 25,000uF. If you believe that a local decoupler changing from 3.3uF to 10uF will have a significant effect on the bass, dream on!
 
"If you believe that a local decoupler changing from 3.3uF to 10uF will have a significant effect on the bass, dream on!"

Deaf?

Guys with EE degrees are the worst.

I sold this stuff for years, I have also been paid to design it.

You're free to do whatever you want.
 
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Guys with EE degrees are the worst.

Hey, I'm a EE and I resemble that! :p (Not really)

I have a buddy that I grew up with and went to university with. We both studied EE. He never designed audio equipment too much and I have been designing and building audio equipment since I was 15 (built my first working amp at 10 it wasn't very good but it worked).

He's a complete numbers junkie. THD + N is the end of the story for him. Of course when you're driving speakers there's a whole world of issues that need to be addressed, but never mind that.

I was gloating to him about how happy I was about repairing my old Nakamichi Stasis amplifier. He asked how much I had into it and I said around fifty bucks. He scoffed and said I could buy a brand new Sony Receiver fro $100 that would blow my old faithful out of the room. You see, My Nak has 55 watts / channel 0.1 % THD + N. The Sony has 100 watts / channel .001 % THD + N. Case closed!

Of course, the Sony no doubt has a run of the mill amplifier topology. The Nak is a two stage amplifier; the output stage is a unity gain Sziklai buffer. This isolates the voltage feedback loop from nasties generated by the speaker and output stage, which reduces higher order harmonic generation and IM distortion. So while this topology shows higher THD + N and lower damping when subject to industry standard tests, it produces much less distortion (especially the nastiest kinds) when actually driving a speaker. It also has enough current gain to not collapse under severe loads. And it's rated 14 amps continuous / channel to boot. But never mind all that - THD + N!

I'll end my rant by saying that I bought one of those Sony receivers around 20 years ago to replace an amplifier I built decades earlier from mostly spare parts, after it (and my speaker) went up in smoke. It was cheap and had amazing specs too; but to make a long story short, it was a huge disappointment. It just sounded thin and wheezy, and shut off after a couple of minutes if played too loud.

Specifications make an important and convenient target for designers, but there is so much more to good design than industry standard specifications. And they way they tout specifications in the marketing of audio products is a big steaming pile of horse-hockey in my opinion.
 
From this thread:

"you might find that after that you get a sense of better impact , better transients and cleaner high ... don't expect miracles if any improvement should be marginal but audible "

or alternatively

"The value of this electrolytic will change the character of the bass more than you know."

Yes, all this engineering is a waste of time - just make it up as you go along! :D
 
Yes Mr. Forrest, there's a lot of bull hockey in hi-fi. Personally I would never claim that changing a bypass capacitor in a low impedance circuit from 3 uF to 10 uF would improve bass. However optimal bypassing can make a difference in transient response and ultimately stability. Of course this will never show up in conventional industry tests unless it is so poorly implemented that its effects are obvious.

Here is an article showing actual measurements of higher order harmonic profiles of various configurations. The Sound of the Machine All this is conveniently not addressed by the industry standard measurements.

Nevertheless, I do acknowledge that these industry standards are very important benchmarks, both for engineers and to a lesser extent consumers. But we should be honest about what they are, and what they aren't.
 
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