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Older ROTEL power amps; suitable for big ESLs?

Mike Reed

pfm Member
Have been looking at the various older Rotel power amps (870BX, 970BX,971BX etc. + RB-06 and RB-03).

Apart from the 870BX at (supposedly) 100 w.p.c., the others are either 70 or60 w.p.c. Now, the electricity usage for some, which is really high, seems to suggest class A, but I think not. Regardless, they all seem okay down to 4 ohms, which is the minimum impedance for my 2905s (4 to 20?).

Damping factor is given (around 200) but feedback figures are not, and I believe highish feedback is recommended for ESLs.

Any thoughts, as I want to try s/s power with my EAR 912 pre. and rest up/service my 509s?

For a fair bit more money, I could go Quad 909 which is, I imagine, guaranteed to suit, whether good or not.
 
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Stock answer I've already given elsewhere 3 times this week.... You do not need to choose amps to suit speakers or vice versa unless we're talking SET's and 103dB/W horns and such like or a few other odd situations. further, there is generally NOT a house sound and amps such as named above may well have been designed 5-10 years apart by different engineers and may be as different from one another as Quad are from Rotel. You can ignore damping factor and feedback here.
 
Have been looking at the various older Rotel power amps (870BX, 970BX,971BX etc. + RB-06 and RB-03).

Apart from the 870BX at (supposedly) 100 w.p.c., the others are either 70 or60 w.p.c. Now, the electricity usage for some, which is really high, seems to suggest class A, but I think not. Regardless, they all seem okay down to 4 ohms, which is the minimum impedance for my 2905s (4 to 20?).

Damping factor is given (around 200) but feedback figures are not, and I believe highish feedback is recommended for ESLs.

Any thoughts, as I want to try s/s power with my EAR 912 pre. and rest up/service my 509s?

For a fair bit more money, I could go Quad 909 which are, I imagine, guaranteed to suit, whether good or not.
Hi Mike,

I have quite a bit of experience with the older Rotel amps. The RB-870BX definitely puts out a healthy amount of power and isn't phased by lower impedance curve drops. In fact, these are well capable of destroying most things, including one's hearing should one be of the 'head banger' sort (not us, obviously).

My only concern here would be with the advancing age of 870BX. These aren't difficult to service but are likely to have fallen off somewhat over the decades. Not that you may care, mind, should one of these be nothing more than a temporary stand in. Regardless, a slightly later 971BX, or perhaps one of those RB-0x models would be more likely to be up to full spec. Either way, I think you will be surprised by how good any of these are for the money. Ear good, if not exactly EAR good.
 
I have found that the lowest distortion possible suits my 989's.
Though my Neurochrome mono amps improved when converted into bridged mode giving around 100WPC each.

Total Harmonic Distortion 0.00033% 1 kHz, 110 W, 4 Ohm.

I'm hoping to try Benchmark AHB2 one day.
 
I have found that the lowest distortion possible suits my 989's.
Though my Neurochrome mono amps improved when converted into bridged mode giving around 100WPC each.

Total Harmonic Distortion 0.00033% 1 kHz, 110 W, 4 Ohm.

I'm hoping to try Benchmark AHB2 one day.

Utterly irrelevant. Do you think you can hear the difference between 0.033% and 0.00033% THD?

After designing and building many many amplifiers over the years I can assure you that once measured performance meets a certain minimum spec then making things ten times better makes little or no difference in most cases.
I still strive to obtain very good measured performance from my designs as "best practice" and for the intellectual satisfaction of it but one of the best amplifiers I've ever heard had about 0.2% THD and a damping factor of about 4... "Le Monstre" to be precise.
 
Utterly irrelevant. Do you think you can hear the difference between 0.033% and 0.00033% THD?

After designing and building many many amplifiers over the years I can assure you that once measured performance meets a certain minimum spec then making things ten times better makes little or no difference in most cases.
I still strive to obtain very good measured performance from my designs as "best practice" and for the intellectual satisfaction of it but one of the best amplifiers I've ever heard had about 0.2% THD and a damping factor of about 4... "Le Monstre" to be precise.

What from a technical point of view could be the reason behind it sounding so good? The minimalist circuit? The monstuous PSU with massive capacitance and huge transformer? Both, or something else?
 
Thanks, gents. Point taken, Craig, about the age thing, though I'd no idea which models superseded which and when. The 870 BX is recommended (and more powerful) but I have read that the older models are better built (though that'll be age relative, I guess.

Anyway, seems to be, from the above, no prob. with ELS/cheap Rotel synergy. Never liked old Quad stuff, but maybe the 909 is the default choice here (secondhand, of course). Jez, damping factor/feedback irrelevance noted; a friend suggested that highish feedback would be more beneficial for ESLs.
 
What from a technical point of view could be the reason behind it sounding so good? The minimalist circuit? The monstuous PSU with massive capacitance and huge transformer? Both, or something else?

As I've mentioned a few times before it's a complete mystery. Nobody knows. If we knew why a selection of amplifiers with similar and all "more than adequate" measured spec sound so different then we could assure that indeed all amplifiers sounded fantastic!
 
Thanks, gents. Point taken, Craig, about the age thing, though I'd no idea which models superseded which and when. The 870 BX is recommended (and more powerful) but I have read that the older models are better built (though that'll be age relative, I guess.

Anyway, seems to be, from the above, no prob. with ELS/cheap Rotel synergy. Never liked old Quad stuff, but maybe the 909 is the default choice here (secondhand, of course). Jez, damping factor/feedback irrelevance noted; a friend suggested that highish feedback would be more beneficial for ESLs.
There is a pattern with the model numbers that aligns them with vintage, Mike.

Simply put, 8## are '80s, 9## are 90's, and 0# are noughties.
 
Have been looking at the various older Rotel power amps (870BX, 970BX,971BX etc. + RB-06 and RB-03).

Apart from the 870BX at (supposedly) 100 w.p.c., the others are either 70 or60 w.p.c. Now, the electricity usage for some, which is really high, seems to suggest class A, but I think not. Regardless, they all seem okay down to 4 ohms, which is the minimum impedance for my 2905s (4 to 20?).

Damping factor is given (around 200) but feedback figures are not, and I believe highish feedback is recommended for ESLs.

Any thoughts, as I want to try s/s power with my EAR 912 pre. and rest up/service my 509s?

For a fair bit more money, I could go Quad 909 which are, I imagine, guaranteed to suit, whether good or not.

I have heard 509 Anniversarys into 989s at a friend's house many times. Also having mixed and matched with other amps and speaker combinations I would say that you have a very synergistic combination there (assuming 2905s sound similar to 989s in the treble). The 509 has "shiny" treble which is OTT with NS1000s but wonderful with 989s.

I have an old Rotel RB850. Several of the newer Rotels use the same circuit, which was originally a Stan Curtis design. IMO these amps do have a house sound... they have some design similarities with the same era Naim and share some of the slight upper mid forwardness and "PRATy" sort of sound. Whether this will work as well as the 509s with ESLs I don't know. Electrically it should be fine, the circuit has good stability into capacitive loads and plenty of spare SOA in the output transistors. The Rotels are highish feedback amps and run at low bias levels i.e. only class A at very low levels.

I have also owned Quad 606, 909 and QSP. The QSP is the best of the bunch - slightly cleaner sounding and tighter bass than the 909. The 909 has a horrible far-too-bright blue LED!
The Quad amps are quite smooth sounding (and a class or 2 above the Rotels). I haven't heard these with ESLs though.
The down-side of the Quad amps is the high gain. You might end up only being able to get to 3, not 11, on your volume control.
 
I have heard 509 Anniversarys into 989s at a friend's house many times. Also having mixed and matched with other amps and speaker combinations I would say that you have a very synergistic combination there (assuming 2905s sound similar to 989s in the treble). The 509 has "shiny" treble which is OTT with NS1000s but wonderful with 989s.

I have an old Rotel RB850. Several of the newer Rotels use the same circuit, which was originally a Stan Curtis design. IMO these amps do have a house sound... they have some design similarities with the same era Naim and share some of the slight upper mid forwardness and "PRATy" sort of sound. Whether this will work as well as the 509s with ESLs I don't know. Electrically it should be fine, the circuit has good stability into capacitive loads and plenty of spare SOA in the output transistors. The Rotels are highish feedback amps and run at low bias levels i.e. only class A at very low levels.

I have also owned Quad 606, 909 and QSP. The QSP is the best of the bunch - slightly cleaner sounding and tighter bass than the 909. The 909 has a horrible far-too-bright blue LED!
The Quad amps are quite smooth sounding (and a class or 2 above the Rotels). I haven't heard these with ESLs though.
The down-side of the Quad amps is the high gain. You might end up only being able to get to 3, not 11, on your volume control.

Ones based on the same design will but what I meant was that Rotel amps over say a 30 year period may use a variety of designs by different engineers... or not... if they have stuck to one basic design even until recently.
 
I would say that you have a very synergistic combination there (assuming 2905s sound similar to 989s in the treble). The 509 has "shiny" treble which is OTT with
Whether this will work as well as the 509s with ESLs I don't know. Electrically it should be fine, the circuit has good stability into capacitive loads and plenty of spare SOA in the output transistors. The Rotels are highish feedback amps and run at low bias levels i.e. only class A at very low levels.

You are right, as the 2905s are very similar to, but better than, the 989s and I'm aware that any departure from my combo is a wee bit tricky. However, one 509 is down (keeps blowing the 2A mains fuse) and possibly t'other may need to go back to EAR as well, but UPS buggered up my outer big box so I'm mentally shelving this until I or someone can drive them there. The 912, also with a fault, will need to go back next week. Nice to know about the feedback figure, S-Man, and your comments re, Quad. My 912 shunts out 5V, and wasn't aware the 909's input sensitivity was low (think that's what you mean).

I had seriously toyed with the idea of going to a quality s/s (Gamut area) and have been Google researching for months, but this kind of stereo chassis power amp doesn't come along used very often. Thought if I could take a punt on a cheapo Rotel, or slightly dearer Quad, I could (a) see what valve pre. into s/s power does, and (b) buy time until the lockdown clears or I can sort out the 509(s) transport container; they were only serviced 30 months ago, so I'm a bit pissed off at this; 3 x serviced in 8+ years until May '18.
 
Just to clarify...
The Quad amps have high input sensitivity = lots of gain. This means that when using preamps with gain, it is likely that you will be restricted to the 1st quarter of the volume control.
 
Why are Quad ESL's so highly regarded.


Because the mylar film has virtually no inertia and moves only a tiny distance, it adds almost no distortion to the audio signal (less than 0.1%) whereas conventional magnetic drivers add up to 3.0% distortion under normal drive conditions, and up to 10% distortion when driven hard.

Some good information here
http://www.meridian-audio.info/public/austquad[1616].pdf

Do you think you can hear the difference between 0.033% and 0.00033% THD?

So yes I can, especially as my digital source has only 0.000069 % THD+N.

Ironically some of the most noticeable differences can be heard when artists add distortion effects to music.
After chasing down distortion in the playback chain, rather than an indistinguishable fuzz, the distortion becomes a clear separate layer.
 
The Quad amps have high input sensitivity = lots of gain. This means that when using preamps with gain, it is likely that you will be restricted to the 1st quarter of the volume control.

Have just checked and the 909 input sens. is 0.775 V and the 405-2 on which it's based, 0.5V, so yes, higher than a lot I've looked at, which tend to be 1V plus. Whether my 5V out would cause prob's with vol. control flexibility I know not. The 912 pre,'s vol. pot tends to be very gradual (with my 509s), where 12 o'clock is a decent but not overloud level. My previous Naim pre. had a more aggressive pot with my 509s and with the 135 mono's which preceded them.
 
I have found that the lowest distortion possible suits my 989's.
Though my Neurochrome mono amps improved when converted into bridged mode giving around 100WPC each.
ESLs have awkward load impedance curves.
An amplifier with good distortion at 10 kHz and truly stable, can sound a lot better than another that has gone for a couple more zeros distortion and is unstable into some loads.
 


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