Repair and modifications of Audiophile AB amp?

[fastline]

I first would like to ask if anyone knows of a popular forum where board level amp mods are discussed? Not to insult anyone here at all, but it would seem that very high end amps live in a world all their own, where designers put their special signature on them....lol

I have a high end system and always fought with 60hz noise in this amp. the amp is a huge, 100# tank with 400wpc at 8ohms. Long story short, I bought the amp barely used, but fedex dropped it. It was totaled technically so I got it for free. I was able to repair the damaged channel board with point/point wiring because it broke some traces. This amp is a true dual mono block design. Basically two discrete amps in one case.

I realize everyone will say "it's ground", and it might be, but I have tried about everything. Even with no inputs plugged in, and the amp floated, it still makes noise. An amp of this grade should have no detectable noise. The SNR is 125db.

But beyond this, I have never been 'blown away' with the sound of the amp, and it seems other audioheads feel the same. There is a bit of grain or fuzz to it, especially at higher listening levels. I want to dig into things a bit to learn if I can make any reasonable upgrades that could help it, such as better capacitor banks, or even more capacitance. To be right honest, I will need to walk down education lane first, but I did do some passive crossover mods on my speakers. in that case, part of the mod was removing the penny cheap ceramic caps, and replacing with audio grade, low ESR film/foil caps. Other mod was tweaking resistor values to get the top to roll off a bit more.
I know at the $2000 price point of my amp, it was 'still' considered a budget minded amp, and corners were cut. But i am hoping I could spot some good upgrades. I have heard from several people that have compared this amp to some of the best, and concluded that it lacks 'something'.....

Now, I do believe this is strictly an AB amp, in which some say a class A is the jam, but they consume massive idle power? That I didn't know. I have never heard of a class D being able to perform at hi-fi levels. As well, I know many are all about tubes, but I want more power and efficiency than tubes can provide I think. But I could entertain the idea.

 

[AnalogKid]

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If you can hear it, you can see it on a scope. Scope shots of the speaker voltage *across the speaker*, both triggered and line-locked.

With no input signal, does the hum/noise increase and decrease with the volume control, or is it relatively constant?

Make / model of the amp?

Schematic?

ak

 

[fastline]

Good point about a scope on the output. there is no volume control. This is only an amp.

No schematic for this amp. Probably could have got it before Rotel got bought up. this is a mod1090. Honestly, I am sure we can figure out the noise issue, but I might not bother if there is no way to improve the sonics. However, I was recently checking a vid by an amp designer of 30yrs and seems we might want to bench mark the amp with a simple resistor so maybe I can see what I am hearing? In other words, stop the BS about 'hearing' better sonics, and actually quantify them?

 

[rjenkinsgb]

Service manual?

Elektrotanya for electronics experts

Electronics forum for electronics experts to repair defective appliance.

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[Nigel Goodwin]

Good point about a scope on the output. there is no volume control. This is only an amp.

No schematic for this amp. Probably could have got it before Rotel got bought up. this is a mod1090. Honestly, I am sure we can figure out the noise issue, but I might not bother if there is no way to improve the sonics. However, I was recently checking a vid by an amp designer of 30yrs and seems we might want to bench mark the amp with a simple resistor so maybe I can see what I am hearing? In other words, stop the BS about 'hearing' better sonics, and actually quantify them?

Unfortunately the so called 'audiophile' world is full of crazy people who imagine things, and they use pointless words to try and justify themselves - talking about 'better sonics' is a minor example

Generally the differences they imagine can't be measured, and can't be proven.

As far as I'm aware none of these people have ever been able to quantify their claims in double blind tests, which rather shows they don't exist - but it doesn't stop them sprouting rubbish all the same.

Your case is different though, I wouldn't class Rotel as 'audiophile', it's just a mid-range manufacturer - but it shouldn't buzz, and if you can hear a buzz from the speakers you should be able to see it on a scope.

First off, does it buzz with nothing whatsoever (other than speakers) connected to it? - I'm presuming it doesn't still buzz without the speakers connected? (which would be a mechanical buzz).

Assuming it does buzz through the speakers with no input, does it buzz on both channels.

 

[AnalogKid]

we might want to bench mark the amp with a simple resistor so maybe I can see what I am hearing? In other words, stop the BS about 'hearing' better sonics, and actually quantify them?

Duh.

ak

OK, a bit snarky; couldn't help myself..

 

[fastline]

Nige. Yes, has noise through speakers with no inputs at all. Both speakers, and no noise if speakers are disconnected.

Not real sure if I should chase the noise from the output or start at the DC buss. I have heard the transformers in the linear PSU can cause this, but I have also read of amps costing 2x this one doing the same.

 

[Nigel Goodwin]

As it's on both channels, then it's got to be something that's common to both channels - so that's drastically reduced your options.

How loud is the buzz?, it's a monstrously huge amplifier, so a tiny buzz good potentially be audible in a quiet room with efficient speakers, while being so far below full output that it's within specification.

 

[fastline]

The noise would only be audible in a quiet room without anyone talking, but anyone with ears would hear it. Even though you don't consider this a high end amp, they are commonly compared against Krell, Crown, Aragon, etc, and in professional listening rooms, the pros have never said, "it makes noise". They are commonly used in home theaters in which the noise floor is one of the most important aspects regarding THX and Dolby quality systems.

As well, there is an NAD amp running in the exact same setup, at the same time, quiet as a church mouse.

My honest opinion is the noise is not normal. It is possible the designers did not cover all their bases regarding killing all sources of buzz, but I have heard of much more expensive amps doing it, but they are usually resolved with improved grounding.

What I commonly do in ground testing for industrial equipment is test for potential to ground, which would confirm that something needs better ground, but I have yet to find such concerns here.

My concern is I have read of a MUCH more expensive, brand new amp giving a little buzz, and eventually went poof. Of course most users don't dig into them so I have no idea how that happened. I can't imagine the DC buss being dirty with the massive capacitors in it, but who knows. We all know capacitors are quiet the weak link in electronics.

 

[AnalogKid]

Buzz is different from hum. While buzz might be at 60 Hz or 120 Hz, it has significant high-frequency content. Unfortunately, gigantic capacitor suck at bypassing high frequencies. I don't see any ceramics or other small value capacitors on the main power rails, nor on the input opamps, so something to try is to add some. I would start with a 0.1 uF and a 2.2 uF in parallel; both ceramic because that is what I already have. A high-quality, low-inductance film type also would work for a trial.

When you say the noise is there with no signal, do you mean with open inputs, the inputs shorted to GND, the inputs connected to a source that is turned down all the way, something else - ?

Also, is the noise on both channels exactly the same amplitude, pretty much the same, or is one channel significantly more quiet than the other?

ak

 

[fastline]

No signal just means nothing connected to inputs. Connected or not, the noise does not change at all. It seems like I need to get the scope on the outputs to get some hard data on this.

Analog, thanks for looking into the circuit. I will do a little testing. As for the service manual, is that link a trusted site? I am too leery to load that just yet, but certainly would like to get the info.

 

[audioguru]

None of the home hifi amplifiers I purchased and none of the amplifiers I designed and made produced any hum or buzz even if my ear is against a speaker. I never had a class-A house heater amplifier that probably produces a hum or buzz because of its very high DC current drain even when it is not playing.

Maybe your class-AB amplifier is wrongly operating in class-A which creates buzz or hum because a part is broken?
Is it hot even when not playing?

 

[Nigel Goodwin]

No signal just means nothing connected to inputs. Connected or not, the noise does not change at all. It seems like I need to get the scope on the outputs to get some hard data on this.

The amplifier input is relatively low impedance, so shouldn't pick up hum with nothing plugged in. You could always short the inputs across the plugs to make 110% sure.

Analog, thanks for looking into the circuit. I will do a little testing. As for the service manual, is that link a trusted site? I am too leery to load that just yet, but certainly would like to get the info.

Yes it is, it's used here all the time.

 

[unclejed613]

since the amp was dropped, it's possible you either missed a ground trace in your repair, or there's another broken ground trace elsewhere in the amplifier. sometimes a break can be very small, and not easily visible. or a through-hole component broke off it's solder pad from the circuit board, and those can be difficult to see.

 

[unclejed613]

Maybe your class-AB amplifier is wrongly operating in class-A which creates buzz or hum because a part is broken?
Is it hot even when not playing?

if that were the case, it would be "double-ended" class A, and the power supply hum would cancel out... i'm sure you are thinking of the hum that comes from a single-ended class A output stage.

 

[fastline]

It seems I will need to collect some measurements and possibly get it opened up to find the noise issue.

However, I feel pretty confident we can solve that. And assuming that is fact, how does one go about 'improving' an amplifier? I am not real certain what components actually contribute to the sound 'quality'. I realize some EEs balk at things like this, but I have been around amps, including guitar amps, all my life and there are certainly amps that sound better than others, even within their own class.

What I am not sure about is if the actual 'circuit design' contributes to this, or the actual components? I realize quality components are best but in the case of my loud speakers, the OEM opted to use cheap ceramic caps over expensive film/foil. I can tell you for certain I 'heard' the difference, but I am not going to say it is the quality of the caps alone that did this. Possible a sweeter crossover point, or similar.

I guess what I am saying is I use my ears, not my pocket book, to determine what sounds good. I have a $200 guitar that, unplugged, hands down sounds better than a $2000 guitar. No one will prove otherwise, but there are obvious quality factors that annoy me on the cheapy.

But if we dig into this amp and say the DC buss voltage being boosted could offer better headroom, that is great, but probably not practical as a simple 'mod'. The only way I can describe the audio is a bit 'abrasive'. A shrill or fuzz to the audio. Some might say it is the speakers, but this amp has been called out running $20k speakers. I do not know how to relate 'warmth' and 'smoothness' in terms of circuits. I would LOVE to visually see the different on a scope so I could understand this. I run horn speakers, which can be painful to hear if fed the wrong watts. What I hear with higher listening levels is compression, or lack of headroom. Basically you can hear the amp maxing out. No more love to give. I sort of question how they got the output RMS ratings on it because I was less than impressed.

I realize some might say it is broke, so fix it, but I don't think that will improve sound. I have run it for 10yrs. Just trying to decide my next step. I have heard insane good pro audio that probably cost a fraction of my setup. I think home audio guys pay a premium for cosmetics. I just want quality sound.

 

[audioguru]

"Shrill" sound is a very simple turn up the treble and/or turn down the bass.
"Warm" is simply turn down the treble.
"Abrasive" or fuzz is simply adding distortion.

Most guitar amplifiers and speakers have lots of distortion, no highs and no lows. They are far from being hifi.

 

[fastline]

"Shrill" sound is a very simple turn up the treble and/or turn down the bass.
"Warm" is simply turn down the treble.
"Abrasive" or fuzz is simply adding distortion.

Most guitar amplifiers and speakers have lots of distortion, no highs and no lows. They are far from being hifi.

I think what I notice in the Rotel is the sound certainly changes with volume, but I also realize many people are tone deaf, and probably would never know quality audio. I also realize we are starting to split hairs here, but I have been to concerns where it is insane loud, yet the sound is effortless and clean. maybe I just need 4 of these amps! lol..

As to guitar amps, obviously distortion can be a really sweet sound....if in phase off tubes. I am mostly referring to acoustics but I do very much realize that "good sound" can be be highly subjective. I recall playing a $20k Martin guitar and telling the guy, I don't like the sound..... He was beyond puzzled. Turns out, I like a little more resonance out of Taylor guitars.

But in reproduction audio, I think we are trying to get to as close to production as possible. What is cool is I can record something with a guitar, then work through steps to replay that on my system. This has helped me in figuring things out.

 

[Nigel Goodwin]

I think what I notice in the Rotel is the sound certainly changes with volume, but I also realize many people are tone deaf, and probably would never know quality audio. I also realize we are starting to split hairs here, but I have been to concerns where it is insane loud, yet the sound is effortless and clean. maybe I just need 4 of these amps! lol..

As to guitar amps, obviously distortion can be a really sweet sound....if in phase off tubes. I am mostly referring to acoustics but I do very much realize that "good sound" can be be highly subjective. I recall playing a $20k Martin guitar and telling the guy, I don't like the sound..... He was beyond puzzled. Turns out, I like a little more resonance out of Taylor guitars.

But in reproduction audio, I think we are trying to get to as close to production as possible. What is cool is I can record something with a guitar, then work through steps to replay that on my system. This has helped me in figuring things out.

Differences in HiFi amplifiers are generally VERY small, it's trivial to make an excellent audio amplifier, with incredible performance - and you're unlikely to be able to pick different amps out in double blind tests.

Guitar amps are completely different, you don't want HiFi, you want distortion, both harmonic and frequency distortion, plus a poor frequency response - added hum and noise is probably good as well. This is why musicians often swear by a particular amplifier, and usually play multiple guitars at gigs, for the specific sound they want for different songs.

 

[fastline]

I feel like I might have got played then because I missed a sale recently of a 5000W amp at Bestbuy for $299.....