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Ross System RG-10 Guitar Amp

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enj99

New Member
I got this amp 2nd hand and it has no output. Somebody has had the circuit out and cut off the ground prong and 2 screws were missing for mounting the pcb into the chassis. The power supply works. The power transformer is center tapped with a DC full wave rectified and filtered output of 32V. The unit uses 5 4558 opamps and a TDA2003 amp. I am suspecting that somebody may have installed the TDA2003 ic in replacement for something else because the TDA2003 operating voltage is 18V max and it is connected directly to the 32V. The board is laid out for a 5 pin power amp ic. I am not able to find a schematic for this but have spent hours making up my own. Any help with this from any of you with experience would be much appreciated.
 

enj99

New Member
I'd like to add: The TDA2003 VCC is connected directly to the Rectifier output of 32VDC. The -VCC of the TDA2003 is connected to Ground. The 4558's are all connected to the same ground as the TDA2003. There is no negative supply. The 32VDC is divided in approximately half with resistor voltage dividers to power the TDA2003. I'm thinking now, because this is the actual design of the PCB and 16V is less than the maximum supply voltage for the 4558's, I'm suspecting that someone put the wrong 5pin audio power amp ic in.
 

enj99

New Member
PROBLEM SOLVED! I couldn't hold my breath any longer for some help here but thank you all anyway just incase any of you did actually read this and begin some thinking. I installed a TDA2050 and it works awesome. The 2050 can use either a single supply or split supply, and it's operating voltage range goes up to 50VDC. I hope this thread will be useful for someone else.
 

HiTech

Well-Known Member
I read your post but couldn't find an online source of detailed service info., so I figured why reply. BTW, that model "was" all tubes.... has it been revamped to be all solid state now-a-days?
 

maheshm

New Member
I have a Ross Systems RG-10 Tube Blaster too - but it makes a constant hiss/scratch s

I too just got one of these amps from eBay.

It makes a scratching/hiss sound constantly without any gear plugged in.

All the pots work without any crackling. Changing the master level changes the loudenss of the scratch/hiss. At zero its basically quiet (a little hum only).

I plugged in headphones, and the scratch/hiss came through that as well - so its not a speaker problem.

With the treble low, the scratch hiss goes down to almost nothing.

Strange situation is that, after the amp has warmed up for a few hours, it the scratch/hiss goes away.

Any thoughts on what may be wrong?
 

unclejed613

Well-Known Member
Most Helpful Member
how loud is the hiss?..... does it go away when you plug something in? is this a tube or SS version? or a hybrid (has tube preamp and SS output)?
 

maheshm

New Member
how loud is the hiss?..... does it go away when you plug something in? is this a tube or SS version? or a hybrid (has tube preamp and SS output)?

Thanks for the fast response.

The hiss doesn't go away when I plug something in. I have used contact cleaner quite liberally on the pots.

Basically, the hiss seems to be generated before it reaches the amplification controlled by the Master. I can reduce the hiss by setting 'treble' down to zero.

This is the SS version. I can't see any tubes.
 

unclejed613

Well-Known Member
Most Helpful Member
most likely you're hearing the 4558 op amps. as i asked how loud is the noise? input preamps can create some noise and nothing actually be wrong with them. the real way to know for sure is to measure the noise voltage at the speaker and compare it to what the full power voltage would be at the speaker, which would give you a ratio. the ratio can be converted to whatever flavor of dB they used in the spec sheet for the amp and compared against the specs for the amp. being a small practice amp, it's probably not even in the spec sheet... RC4558 op amps are spec'ed for 10nV/√Hz. so for a gain of 100 input stage, at 1kHz, the noise voltage would be 31mV, and in a 10W amp, the voltage gain is probably about 10, so that would be 310mV at the speaker. that definitely would be audible. if the op amps are old or there are large value carbon composition resistors used in the input circuit, you could have a lot more noise than the basic noise floor of the op amp. also be aware that semiconductor junctions begin to develop more noise before they fail, so you may have an op amp in the process of failing (NASA uses this fact in their reliability program).
 

maheshm

New Member
most likely you're hearing the 4558 op amps. as i asked how loud is the noise? input preamps can create some noise and nothing actually be wrong with them. the real way to know for sure is to measure the noise voltage at the speaker and compare it to what the full power voltage would be at the speaker, which would give you a ratio. the ratio can be converted to whatever flavor of dB they used in the spec sheet for the amp and compared against the specs for the amp. being a small practice amp, it's probably not even in the spec sheet... RC4558 op amps are spec'ed for 10nV/√Hz. so for a gain of 100 input stage, at 1kHz, the noise voltage would be 31mV, and in a 10W amp, the voltage gain is probably about 10, so that would be 310mV at the speaker. that definitely would be audible. if the op amps are old or there are large value carbon composition resistors used in the input circuit, you could have a lot more noise than the basic noise floor of the op amp. also be aware that semiconductor junctions begin to develop more noise before they fail, so you may have an op amp in the process of failing (NASA uses this fact in their reliability program).


Thanks for the advice. The noise is significantly more than what you would expect under normal circumstances and it is very loud compared to the actual audio signal.

How can I easily measure the noise voltage at the speaker? I only have a digital multimeter that probably isn't very accurate at the mV readings.
 

unclejed613

Well-Known Member
Most Helpful Member
digital DMMs are okay to measure with. if the noise is louder than, say a guitar signal (any guitar), then you do have a problem with your input stage. hiss like that is abnormal. you could probably trace the signal with an oscilloscope and easily find the source of it. it's most likely the input preamp chip, since it's usually the one with the highest gain.
 

maheshm

New Member
digital DMMs are okay to measure with. if the noise is louder than, say a guitar signal (any guitar), then you do have a problem with your input stage. hiss like that is abnormal. you could probably trace the signal with an oscilloscope and easily find the source of it. it's most likely the input preamp chip, since it's usually the one with the highest gain.

Thanks - I will have a look. The noise is almost as loud as the guitar signal, but not louder. Its loud enough to be distracting and annoying. I will investigate further.
 

unclejed613

Well-Known Member
Most Helpful Member
most likely a dual op amp, such ad an RC4558 (or NJM4558, etc...).... replacing a 4558 with a TL072, should take care of it...
 
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