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Old 23rd September 2014, 01:44 PM   #1
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Default Need help for modifying an amplifier

Hey everyone.

A month ago a brought home a dead Yamaha 5.1 reciever, the model is RX-V800. It`s a 100w per channel receiver. However the micro-computer was totally dead, but I did salvage the power stage, and the tone control circuit.

Mostly I will use it with a 3 channel active crossover to power a pair of 2 way speakers and a subwoofer but also want to use it as a stereo amp with tone control when I need to.

The amplifier works flawlessly so does the tone control. I also hocked up the Bass Extension and a tone control bypass switch but have the following issue. If i turn off the amplifier with the tone control bypassed there is no on/off thump (the delay and speaker protection relay circuit doesn`t work yet, so i bridged the relays) and after turning it back on I can safely turn on and off the tone control.

However with the tone control on there is a very loud thump when I turn the amplifier on or off, and also when i switch the tone control on and off afterwards. The switching thump between tone on and off only occurs when I turn the amp off with the tone control on.

Sorry if it sounds a bit confusing.

The tone control has both output and input coupling capacitors so I don`t think that`s the problem, however the amp in it`s original form had an uPC4570 opamp at the tone control signal input that acted more as a voltage follower then a preamp.

Any help is appreciated.
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Old 23rd September 2014, 04:22 PM   #2
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Quote:
Originally Posted by partyface86 View Post
Hey everyone.

A month ago a brought home a dead Yamaha 5.1 reciever, the model is RX-V800. It`s a 100w per channel receiver. However the micro-computer was totally dead, but I did salvage the power stage, and the tone control circuit.

Mostly I will use it with a 3 channel active crossover to power a pair of 2 way speakers and a subwoofer but also want to use it as a stereo amp with tone control when I need to.

The amplifier works flawlessly so does the tone control. I also hocked up the Bass Extension and a tone control bypass switch but have the following issue. If i turn off the amplifier with the tone control bypassed there is no on/off thump (the delay and speaker protection relay circuit doesn`t work yet, so i bridged the relays) and after turning it back on I can safely turn on and off the tone control.

However with the tone control on there is a very loud thump when I turn the amplifier on or off, and also when i switch the tone control on and off afterwards. The switching thump between tone on and off only occurs when I turn the amp off with the tone control on.

Sorry if it sounds a bit confusing.

The tone control has both output and input coupling capacitors so I don`t think that`s the problem, however the amp in it`s original form had an uPC4570 opamp at the tone control signal input that acted more as a voltage follower then a preamp.

Any help is appreciated.
Hi,
From the schematic, after the uPC4570 op amps you describe, there is a mute circuit based around Q525, Q526 (for the RL channel).

I am not 100% certain, but it's possible that this mute circuit was originally activated momentarily when switching the tone control bypass (and/or other functions) so as to prevent thumps.

Have you tried enabling the mute circuit, or at least tried forcing it into the mute state before switching? The mute control line is labelled 'FMTM' and originates at pin 68 of IC526 as "/FMTM", which is active low logic level. It is then inverted by Q533 & Q534 before becoming "FMTM".

To test this, cut the trace from the junction of R773/R744 leading to IC526. Connect a wire from R773/R744 junction to a +5V supply to enable audio and to 0V to mute.

Also, where exactly did you connect your tone control bypass switch and is it AC or DC-coupled?
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Last edited by currentflow; 23rd September 2014 at 04:39 PM.
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Old 23rd September 2014, 06:27 PM   #3
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correct !! the mute function is present in tone on and of and if memory serves also when changing sources and may be some other functions

So yes ..make the protection work plus the start up delay

Is useful for thsi an d other reasons in general is very bad to run this amp with no protection at all

The mp is monitoring many things like under over voltage plus overload or short circuit for every CH you are at the risky side !!!
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Old 23rd September 2014, 07:14 PM   #4
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Quote:
Originally Posted by currentflow View Post
Hi,
From the schematic, after the uPC4570 op amps you describe, there is a mute circuit based around Q525, Q526 (for the RL channel).

I am not 100% certain, but it's possible that this mute circuit was originally activated momentarily when switching the tone control bypass (and/or other functions) so as to prevent thumps.

Have you tried enabling the mute circuit, or at least tried forcing it into the mute state before switching? The mute control line is labelled 'FMTM' and originates at pin 68 of IC526 as "/FMTM", which is active low logic level. It is then inverted by Q533 & Q534 before becoming "FMTM".

To test this, cut the trace from the junction of R773/R744 leading to IC526. Connect a wire from R773/R744 junction to a +5V supply to enable audio and to 0V to mute.

Also, where exactly did you connect your tone control bypass switch and is it AC or DC-coupled?
The audio source/preamplifier board is not in the amp anymore, only the main amplifier and video board as it also contains the amplifiers driver stage, amplifier power supply with the filter caps and tone control is in it. I also had to remake the voltage regulator board for the +-12v power rails. So no mute circuit.

I have cut the traces at the original 6pin bypass switch as it was wired to function as a 1 way switch and I had to separate all the pins from each other, the amplifiers input goes to the 2 center pins of the switch, so basically it couples the volume pots sweeper directly to the amps input or the volume pots sweaper goes to the input of the tone control and then to the amps input. The volume pots sweeper is connected in parallel with the amp and tone control when bypassed

I think the cap is to block any dc from the tone controls output

Last edited by partyface86; 23rd September 2014 at 07:31 PM.
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Old 23rd September 2014, 07:24 PM   #5
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Quote:
Originally Posted by east electronics View Post
correct !! the mute function is present in tone on and of and if memory serves also when changing sources and may be some other functions

So yes ..make the protection work plus the start up delay

Is useful for thsi an d other reasons in general is very bad to run this amp with no protection at all

The mp is monitoring many things like under over voltage plus overload or short circuit for every CH you are at the risky side !!!
OK. Is there another way to remove the thump, old amplifiers did not have any muting circuits and did not produce any loud thumps between switching. I did found an article about thumps when switching sources and one way to fix this problem is to connect a resistor after the coupling cap and ground. Will this work?

And also I am not really worried to operate the amp without the protection relays for now when the amp is connected directly to the source there is very little on/off thump. But yes, I will make a protection and delay circuit. Unfortunately I will have to remake it from scratch as the main microcomputer controlled all the delay and protection functions
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Old 23rd September 2014, 07:43 PM   #6
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Quote:
Originally Posted by partyface86 View Post
The audio source/preamplifier board is not in the amp anymore, only the main amplifier and video board as it also contains the amplifiers driver stage, amplifier power supply with the filter caps and tone control is in it. I also had to remake the voltage regulator board for the +-12v power rails. So no mute circuit.

I have cut the traces at the original 6pin bypass switch as it was wired to function as a 1 way switch and I had to separate all the pins from each other, the amplifiers input goes to the 2 center pins of the switch, so basically it couples the volume pots sweeper directly to the amps input or the volume pots sweaper goes to the input of the tone control and then to the amps input. The volume pots sweeper is connected in parallel with the amp and tone control when bypassed
I was going to suggest placing the tone defeat switch as a "dual-gang single pole changeover" in a position equivalent to IC522 pins 4,5,6, where pin 6 is the common connection.

This receiver could form an interesting project incorporating one or more Arduino Nanos and allow IR remote control of source, volume, balance, DC protection and over-temperature & overload monitoring etc. plus a display.

Quote:
Originally Posted by east electronics View Post
correct !! the mute function is present in tone on and of and if memory serves also when changing sources and may be some other functions

So yes ..make the protection work plus the start up delay

Is useful for thsi an d other reasons in general is very bad to run this amp with no protection at all
Sensible advice!

Quote:
The mp is monitoring many things like under over voltage plus overload or short circuit for every CH you are at the risky side !!!
This could be homegrown with a little effort...
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Old 23rd September 2014, 07:59 PM   #7
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Quote:
Originally Posted by currentflow View Post
I was going to suggest placing the tone defeat switch as a "dual-gang single pole changeover" in a position equivalent to IC522 pins 4,5,6, where pin 6 is the common connection.

This receiver could form an interesting project incorporating one or more Arduino Nanos and allow IR remote control of source, volume, balance, DC protection and over-temperature & overload monitoring etc. plus a display.


Sensible advice!


This could be homegrown with a little effort...
Unfortunately I had taken out a lot of parts out of the preamp section before an in depth analysis of the circuitry, only afterwards did I realize that I could have saved a lot of functions without the need of the main microcomputer, like preamps, input source switches as they are all controlled by analog switch array ICs, etc.

As for Arduinos, I know nothing about them, maybe it would have been easy for you but not for me . My electronics skills is also limited

Last edited by partyface86; 23rd September 2014 at 08:02 PM.
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Old 23rd September 2014, 08:13 PM   #8
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Quote:
Originally Posted by partyface86 View Post
Unfortunately I had taken out a lot of parts out of the preamp section before an in depth analysis of the circuitry, only afterwards did I realize that I could have saved a lot of functions without the need of the main microcomputer, like preamps, input source switches as they are all controlled by analog switch array ICs, etc.
If the discarded sections cannot be salvaged and adapted for re-use, there are some other options:

a) Design your own or use existing proven designs
b) Leave these out and just use a basic input selector switch
c) Obtain kits or pre-built modules to meet your needs
d) Use an external pre-amplifier (or buffer) with source selection

The possibilities are endless.

Quote:
As for Arduinos, I know nothing about them, maybe it would have been easy for you but not for me . My electronics skills is also limited
Arduinos are low-cost microcontroller modules with free development software and programmed via a PC USB connector. There is a huge range of resources and examples to be found online.
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Last edited by currentflow; 23rd September 2014 at 08:28 PM.
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Old 23rd September 2014, 08:38 PM   #9
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what you should keep in mind is that Yamaha and many other companies producing amplifiers of that range use a very generic circuit often way out of specs to produce more headroom and produce more watts on bench tests and peak power ...

The designs often use just a couple of transistors for the output stage and claim often far more than 100W often in voltage above 40volt and often seen even 50 or even more ..

A couple of transistors could be safely operate with no more of 35+35 volts at 8 ohms loads possibly in 4 ohms but not full power and continuously. Anything above that you will need some method to limit , control or secure your circuit against abuse intended or accidental .

In your case the mp is monitoring all these things and will activate the protection when any abuse condition is present

So yes since you can no longer monitor output current you will need protection cause if you push this amp it will eventually fail ...

Kind regards
Sakis
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Last edited by east electronics; 23rd September 2014 at 08:41 PM.
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Old 23rd September 2014, 11:29 PM   #10
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East is right ...

RX-V800 = 55-0-55V 300VA trafo , sanken MNxxxx darlingtons (50w amps) +
2 X 6800/8200uf 63V supply caps.

Amp is designed around IC stages driving the op devices. Sony 5 channel
receivers are similar .

Trafo makes for a good REAL 100-150W subwoofer PS (or 2 X 60w REAL )

It is what it is .

OS
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