Well, the rather noisy harddrive in my computer about 2.5 m away (Deskstar 7K500, a 5-platter affair) is making at least as much of a racket, but the noise still is quite plainly audible to me. We are talking very quiet surroundings and only 3 decades of mileage on those ears. Not sure about frequency response of Jamo D570s, they're not shy of highs for sure. The amplifier in question is an old Onkyo TX-SV636 AVR - 50k pot, 4558 opamp (resistor noise negligible) and total Av = 45 dB. Should be a touch less than 300 µV even.
My point exactly. Ideally, you want to be able to run high-sensitivity horns without any objectionable noise. Though many amps with conventional volume pots tend to get into a range with pitiful channel tracking then... those 20 dB attenuator switches definitely were good for something. In fact, people have complained about bad channel tracking in some models even with normal-sensitivity speakers.
Drawing errors, most definitely - possibly to deter people from copying, or maybe the schematic was just drawn in a hurry. Obviously the feedback line should not connect to the drivers, and those C-B connections may have been intended for some additional components.
Let's not confuse AC noise with DC offset. If you are hearing noise from your amp, that is a totally different set of problems.
Yes, Rotel, and many other brands have intentional errors in the schematics. An experienced tech will have no problem. I am guessing this is some kind of trick to watch for copyright or rip-offs. Seems silly to me.
Trimpots, just like the rest of the circuit, are not stable with temp. A bit of old age, and they can do really bad things, like open. Then you are talking full rail offset. Duck while the woofers come flying at you. No, either do a proper servo, or leave well enough alone. You have been warned.
Part of DIY is to do as you wish anyway and suffer the consequences. You can get luck or you can end up with blown amps and speakers. Your choice. Pay to play. Myself, I am so old that I don't have time to learn everything for myself every time, so I listen tho those who have been down that road before. Want to know how much a set of Exicon MOSFETs for a Hafler cost? That's a hint son.
Yes, Rotel, and many other brands have intentional errors in the schematics. An experienced tech will have no problem. I am guessing this is some kind of trick to watch for copyright or rip-offs. Seems silly to me.
Trimpots, just like the rest of the circuit, are not stable with temp. A bit of old age, and they can do really bad things, like open. Then you are talking full rail offset. Duck while the woofers come flying at you. No, either do a proper servo, or leave well enough alone. You have been warned.
Part of DIY is to do as you wish anyway and suffer the consequences. You can get luck or you can end up with blown amps and speakers. Your choice. Pay to play. Myself, I am so old that I don't have time to learn everything for myself every time, so I listen tho those who have been down that road before. Want to know how much a set of Exicon MOSFETs for a Hafler cost? That's a hint son.
You can use pots to check the said optimum value of course
but you cant use different final values for each channel
as the passive tone control output wich has about 1.5K output
impedance is connected directly to the power amp input.
This mandate the same value for the two channels otherwise
the tonal and volume balance will be set out of symetry
when the tone control is used.
Since you ve got 68mV and 45mV DC offset for each channel
respectively , that is 23mV dc inbalance , reducing to 12K will
surely be optimal only for the former while the latter would need
perhaps 22K , the optimum will surely be to set the said resistor
such that one channel is about -11.5mV and the other one at +11.5mV.
Edit: After checking with pots use the nearby resistors values
since potentiometers are not reliable as pointed above.
As a side note , this kind of symetrical differential topology,
wich i prefer, is very rare and doesnt even account for
0.01% of the amps sold to this date....
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Very rare ???
Every Rotel since around 1990 uses this simple schematic, more output devices for higher output is about the only differences to high power models together with vas cascoding, although it is a patented form of cascoding that rotel uses.
I think this was all taken into account, this amp was designed for low and medium end of the market btw, it cost only 250 pounds back in 1992 but could compete with double priced units from other manufacturers. Its quiter than many so called high end 10 000 euro amps
so I really doubt noise level is such a problem. The design is by Stan Curtis.Very rare ???
What is Rotel in the total amount of amps sold since the early 70s?...
The usual single LTP + VAS + OPS is surely 99.99% of all the produced
amps since the early 70s , and 100% of the eventual chip/thick film amps....
The typical Rotel power amp is a pair of complementary LTPs (no emitter degeneration, crude current source and single ended loads) followed by complementary push pull VAS and output triples with no protectio0n circuits. Oddly there is no rail RC filter to the sensitive bits. The circuit depends on npn-pnp symmetry for offset and to reject rail ripple.
What is Rotel in the total amount of amps sold since the early 70s?...
The usual single LTP + VAS + OPS is surely 99.99% of all the produced
amps since the early 70s , and 100% of the eventual chip/thick film amps....
I really doubt that, Harmon uses dual LTP, Nad for a while and are now back at it again, these amps take a very large slice of total amps sold as they are affordable to most. I bet the figures are more like 80 % if that.
I alone have 7 of various models, whenever I can buy one second hand for cheap I plunge and with a little tweaking these amps sound great.
Friends, with all this information, I'm confused. Please only want to hear music with decent sound, only just want to improve my Rotel to get the most performance. Can be achieved by changing the DC offset and the corresponding capacitors, electrolytic capacitors as well in the preamplifier signal?, Or I just go to a Nad, or another amp. Here an amp like Rotel or Nad this level is used to pay U $ S 500 - 600, and up to U $ S 1250 RA_1062 a Rotel (which for me is a RA-970BX but with remote control). As I like the sound of Rotel with B & W boxes, I'd rather improve the Rotel, but if you say the solution is difficult, I will try to sell it and buy Nad. Greetings Jose.Deshacer cambios
Apart from the said DC offset wich can be quite easily reduced
the only thing it will benefit eventualy , as noted in a post above ,
is to add cheap RC cell to both of the front ends wich will
substancialy improve supply noise rejection.
Set apart those minor mods there s nothing that is worthy
of modification , it s already a very good amp.
the only thing it will benefit eventualy , as noted in a post above ,
is to add cheap RC cell to both of the front ends wich will
substancialy improve supply noise rejection.
Set apart those minor mods there s nothing that is worthy
of modification , it s already a very good amp.
I send you the circuit of the Rotel Ra-970BX im talking about. I think you have confused the circuit with the other Rotel. José.
If you want to see the picture in detail, here is the direct link: http://i48.tinypic.com/qpjp6w.jpg
If you want to see the picture in detail, here is the direct link: http://i48.tinypic.com/qpjp6w.jpg
An externally hosted image should be here but it was not working when we last tested it.
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Thank you very much everyone for the opinions expressed. I replaced the transistors of the differential input of both channels 2SA1016 and 2SC2362 Sankyo ´matched (Q 301-603, Q607-605, Q606-608, Q 604 to 602) and capacitors C 607-608, C 611-612 Rubycon.
Result: Before minus 45 mv in L and plus 78 mV in R.
Now less 20mv in L and R.
Im the 15 mv, which is considered correct.
I also set the quiescent current to 8 mv.
I think I've renovated a Rotel 18 years thanks to you.
Impressive sound better, better and more extensive scene; gaves a little more powerful and more treble clarity. Thanks again, Joe.
Result: Before minus 45 mv in L and plus 78 mV in R.
Now less 20mv in L and R.
Im the 15 mv, which is considered correct.
I also set the quiescent current to 8 mv.
I think I've renovated a Rotel 18 years thanks to you.
Impressive sound better, better and more extensive scene; gaves a little more powerful and more treble clarity. Thanks again, Joe.
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Thank you for the suggestion.
I had -86mv and -105mv and after just replacing R603 and R604 with 22k I am down to -17mv and -35mv. 70mv in each channel.
For such a simple modification, the benefit seems big.
Do you think it's worth trying 18k or 15k ?
I don't understand very well how audio amplifier design works and feedback loops scare me
Is there any trade-off between low DC output voltage and something else when lowering R603/604 ?
I had -86mv and -105mv and after just replacing R603 and R604 with 22k I am down to -17mv and -35mv. 70mv in each channel.
For such a simple modification, the benefit seems big.
Do you think it's worth trying 18k or 15k ?
I don't understand very well how audio amplifier design works and feedback loops scare me
Is there any trade-off between low DC output voltage and something else when lowering R603/604 ?
Friends, after three years, revisit the issue of dc offset correction.
As I said, after replacing the transistors of the differential inputs by Sankyo matched, the amplifier has remained at -20mV in both channels.
As I am on track to replace the power supply capacitors by Nichicon Gold Series 10000/63 and bridge rectification Diodor other 35 A high-speed, also I wish correjir that little offset.
The change of R 603-R604 from 47k to 22k would be appropriate now ?. It does not affect the input impedance ?. Would have to replace C601-C602 4.7 uF Black Gate
As I said, after replacing the transistors of the differential inputs by Sankyo matched, the amplifier has remained at -20mV in both channels.
As I am on track to replace the power supply capacitors by Nichicon Gold Series 10000/63 and bridge rectification Diodor other 35 A high-speed, also I wish correjir that little offset.
The change of R 603-R604 from 47k to 22k would be appropriate now ?. It does not affect the input impedance ?. Would have to replace C601-C602 4.7 uF Black Gate
Thank you very much friends in this forum.
Finally, after three years I have decided to make a change that I had suggested.
Is to bring R 604 and R603 47k to 23k5 (placing another resistor in parallel 47k 1% metal film).
Result: -0.004V offset on both channels (- 4mV).
In short: replacement differential pairs matched by new Sankyo; replacement Number 607 C 608 C by Nichicon mouse; Replacing the variable resistor bias adjustment for multiturn pot; and today I encouraged to take R 603 - R 604 to 23.5K.
Offset result was 45 mV in 68 mV in L and R on both -4 mV.
Bias adjustment is easy thanks to the multi-turn pot.
All thanks to your contributions. Thank you.
![IMG]](/community/proxy.php?image=http%3A%2F%2F%5Burl%5Dhttp%3A%2F%2Fi58.tinypic.com%2F14e1iyr.jpg%5B%2Furl%5D+%5B%2F+IMG%5D&hash=63554f3d4487b8493c49649528bb66df)
Finally, after three years I have decided to make a change that I had suggested.
Is to bring R 604 and R603 47k to 23k5 (placing another resistor in parallel 47k 1% metal film).
Result: -0.004V offset on both channels (- 4mV).
In short: replacement differential pairs matched by new Sankyo; replacement Number 607 C 608 C by Nichicon mouse; Replacing the variable resistor bias adjustment for multiturn pot; and today I encouraged to take R 603 - R 604 to 23.5K.
Offset result was 45 mV in 68 mV in L and R on both -4 mV.
Bias adjustment is easy thanks to the multi-turn pot.
All thanks to your contributions. Thank you.
I've got the Rotel RB-970BX, and wasn't impressed by the approx. 50mVDC at both outputs. I wanted to drive high end tweeters (Seas Millenium 1 inch domes) directly since I'm tri amping with an active crossover, and 50mVDC on a tweeter seems bad. The consensus of the local audiophile group seemed to be around 20mV, before they consider it not good enough. I consulted with the schemo for my Hafler DH-220 to see how they added a DC offset function that would be relatively failsafe, and then built one for each channel on a tiny piece of perf board, which I mounted to the case with an L bracket. I think the reason for the offset is the substantially different DC resistances at the bases of the input transistor pairs. What makes this fix practical is the relatively low Z in the negative global feedback circuit (approx. 12K). That means that a few inches of wire between the main board and my perf board shouldn't be a problem. But do what you can to keep these wires short. So the injection point for this bias voltage is the feedback bases of the input differentials, or the right hand side of C685.
The circuit is a 1K trimpot, with a 1% 40K R off each end going to each pwr supply. There are diodes across the 40K R's that are normally reverse biased, so normally don't do anything. They add protection (during turn-on and off). Off the wiper of the pot, I have a 10n cap to gnd., then a 40K R feeding the injection point of the amp circuit. I'd use half watt metal film R's. Keep these perf boards away from power supply or output wiring. If you're a beginner, you might want to just sell the amp and get a different one that doesn't have this problem. I took a picture of what I did, but can't find it. sorry.
While you're in there, you might add 0.1uF PP caps across the main power supply electrolytics. They will minimize the impedance looking into the power supply, as viewed by the amp circuit, and keep it minimal over time. That's good for phase margin stability.
The circuit is a 1K trimpot, with a 1% 40K R off each end going to each pwr supply. There are diodes across the 40K R's that are normally reverse biased, so normally don't do anything. They add protection (during turn-on and off). Off the wiper of the pot, I have a 10n cap to gnd., then a 40K R feeding the injection point of the amp circuit. I'd use half watt metal film R's. Keep these perf boards away from power supply or output wiring. If you're a beginner, you might want to just sell the amp and get a different one that doesn't have this problem. I took a picture of what I did, but can't find it. sorry.
While you're in there, you might add 0.1uF PP caps across the main power supply electrolytics. They will minimize the impedance looking into the power supply, as viewed by the amp circuit, and keep it minimal over time. That's good for phase margin stability.
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