DC OFFSET AT ROTEL

[sgrossklass]

Originally Posted by AndrewT

My ears are as old as me.
I would have to get a lot closer than 1m to hear 330uVac of 20Hz to 20kHz of noise through 88dB/w @ 1m speakers.

330uVac is ~ -70dBW and would produce ~18dB SPL at 1m.

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.

Originally Posted by AndrewT

But that does not stop me from striving to better 50uVac of noise at the output.

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.

Originally Posted by wahab

The pre drivers have their bases connected to their collectors.

The PNP pre driver and driver have no emitter resistors
and the common point with the emittor resistors of the
complementary pre driver and drivers is connected not to
the output but directly to the inverting input....

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.

[ALPUY]

Dear WAHAB, can instead be placed R603-R604 50K multiturn trimpot and then adjust it so that the DC offset is as little as possible?. I appreciate your opinion. José.
Varying the resistor, the dc offset varies?.

[tvrgeek]

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.

[wahab]

Originally Posted by ALPUY

Dear WAHAB, can instead be placed R603-R604 50K multiturn trimpot and then adjust it so that the DC offset is as little as possible?. I appreciate your opinion. José.
Varying the resistor, the dc offset varies?.

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....

[homemodder]

Originally Posted by wahab

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....

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.

[homemodder]

Originally Posted by sgrossklass

Output transistors have nothing to do with it, it's the inputs. In theory, the npn and pnp inputs (in this dual differential input design) would be exactly complementary and running at exactly the same current - in practice, they're not, and those transistors also exhibit aging to some degree, causing leakage to creep up over time. Even if the transistors in each differential pair were aging equally (there's a significant statistical component involved), npns and pnps would still differ. I'd guess the pnps are more critical.

I am not sure what kind of architecture the AD827/847 uses - I'm guessing an AB push-pull type à la OP467? Or folded cascode like AD826 and AD829? Either way, input impedance nonlinearity should be reasonably low, and transfer linearity more than adequate for the purpose at least. It shouldn't be bothered by high frequencies all that much, nor will capacitive loading upset it according to the datasheet. So it looks like a good match for the circuit here, which upon closer inspection is not too generous with high-frequency input filtering nor does it provide series resistors for the opamp outputs (cheapskates...). Except, well, it's noisy, which is not a good match for a circuit with all its gain after the volume pot.

Calculated output noise is about 330 µVrms @ 20 kHz b/w, which is decidedly average. This kind of noise level produces a clearly audible - if not yet objectionable - noise floor at 1 m from speakers rated at 88 dB / 2.83 V, which is hardly Klipschorn-level sensitivity. (I'm in 4-ohm land, which on average is 3 dB more sensitive.) It is not that uncommon for mass-market integrated amps to exhibit a residual noise floor of 100..150 µV (most any modern-day Denon, Marantz, NAD and the little Pioneers). Even PGA-equipped gear seems to have gotten fairly good if you look at the specs of the latest stereo receivers from Yamaha or Onkyo.

Considering the potential issue of capacitive loading, a TI NE5532 may not be all that happy indeed (a case for a scope). The LM4562 is supposed to drive up to 100 pF at least. Adding 100 ohms in series with coupling caps C505 and C506 (10µ/50 - yeah, it may not look pretty) should get this sorted in any case. At least supply bypassing seems to be good.

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.

[tvrgeek]

Ah, now I know whom I may complement. Thank you Mr. Curtis.

[wahab]

Originally Posted by homemodder

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.

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....

[davidsrsb]

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.

[homemodder]

Originally Posted by wahab

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.