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Revisiting Jim Rogers JR149s

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I’ve just landed an Onix OA21S as it was in lovely condition and at what I felt was a good price. It has to be one of the last ones made having a gold logo and the speaker terminals found on the Onix pre/powers that came out in 1990. I had one of the very first OA21s (not ‘S’) back in the day and loved it, a wonderful product that punched well above its price class. This one looks totally stock and unrestored, I’ve done nothing more than clean it up, reset the bias and check the DC offset (no issues to be seen inside). It sounds just how I remember my old Onix; crisp, tight, punchy, open, refined and far better than it should be for the price!

It is an interesting contrast to the Leak/Audio Synthesis Passion, and in many respects exactly what one would expect between valves and solid state. The Onix is brighter, drier, flatter, more forward and upbeat, the Leak liquid, textural with just beautiful instrumental timbre and three-dimensional space yet no great sacrifice in energy or momentum (it is a surprisingly funky thing). There is less between them than I expected it has to be said, the Onix once again punches well out of its price class. It has been very interesting as I’m comparing a ten watt tube amp against a punchy 40 Watt solid state amp and the Leak doesn’t sound wanting, so I assume I’m not pushing it beyond its envelope. I’m sure I could push the Onix a lot further though. I level matched as best I could with my iPhone sound meter so I’m well within my normal listening level (76-78db average, 91db peaks on good dynamic jazz cuts).
 
I have fond memories of the OA21S. Very nice phono stage.
From memory I found the line input less satisfying but would have been using an early CD player at the time, so perhaps it wasn't the Onyx.
 
As I understand it the OA21S differs from the earlier OA21 in having a passive preamp stage, i.e. it is just a power amp with an input selector and volume knob plus an active phono stage. As such it can’t be clipping with a digital input. Plenty of movement on the volume knob too, I’m listening at about 10-11 o’clock, so clearly designed with CD in mind (a Nait is all done by about 9!). It does have a crisper more forward presentation than the Leak, but I guess that is no surprise, especially given the Leak is stuffed with nice creamy old Mullards. Certainly no hint of grain or edge, though had I set the JR149s tweeter level pots with this amp rather than the Quad 303 I used I may have gone just a tiny nudge back from what I settled on. The top end of the Onix is very good, I suspect a little more dynamic than the Leak, cymbals etc sound very alive and free if a little less liquid and located in acoustic space. I’m using a Marantz SA8005 SACD player which is a very nice and refined player for the cash.
 
I’ve always really rated the Onix, I stuck my first one up against some far more expensive Musical Fidelity kit at a friend’s and thought it was far better, and when I swappd it out for Naim (62/140) I had a real sinking feeling for a long time as I felt there was something lost. It was only by adding a HiCap that I felt I was better than where I’d started from, and that was a combo that cost three or four times the OA21! I bought my first one brand new in a dem against a Nait 1 via Kans. Definitely the better amp to my ears at that time.
 
They pop up now & again, may give one a try next time, sounds like something I would enjoy.
 
Interesting thread! I have a pair of JR149s (s/n 4473 so very early) with no fuse, individual pins on the PCB for the speaker drive units etc..
Having blown a bass driver courtesy of a problem with my amp I'm looking to protect my JR149s with a fuse to prevent another expensive replacement (courtesy of falcon). I'm assuming I'll need an inline fuse the input speaker wire. But does anyone have any guidance as to what ampage? I know the normal tweeter fuse is 1.5A to 2A, but I'm looking for the whole speaker. I'm not sure if it's a good idea to leave it there permanently or not, but certainly once the amp is repaired in case there is another problem (caps in the amp were leaking, I'm replacing those and the mosfets, hopefully nothing else needs fixing).

Had these JR149s since I was a teenager in the 70s although they were lent to a nephew for a while in the 90s and I've only recently started trying to use them again. Looking forward to getting them going.

Thanks,
Jeddy
 
I’d have thought if you recapped and serviced the amp according to the service manual with regards to rail voltage, bias, DC offset etc all should be safe again. I’d be inclined to run any testing with a cheap throwaway speaker rather than the 149s and as such I’d be rather reluctant to modify them. What is the amp?
 
No need to modify the crossover, you just insert a fuse just before the speaker cable from amp reaches the JR149 crossover. Whether or not this will actually protect the speaker from the risks you are concerned about is open to debate, as is whether or not a fuse will have a negative impact on the sonics. I've owned several pairs of JR149s over the years and on one pair I decided to bypass the fuse on the XO with silver wire. Can't say I ever noticed a difference in sound quality!....
 
Thanks. It's a Roksan S1 (originally used with another pair of speakers). Hmmm, servicing the amp... I took it to someone who repairs hifi who took a look and quoted £200-£300 as it needed new caps, not worth it. I opened it up myself and there are 12 caps, all easily replacable so I'm in the process of doing that myself (I've replaced caps in other things e.g. PVR PSUs etc.). No service manual or proper service equipment. The throwaway speaker is a sensible approach. I can probably pick up something cheap and nasty easily enough.

Yes, I wasn't planning on changing the crossover, just add an inline fuse in the speaker cable. I agree with the comments as to debate as to it's use and sonic impact, more of an initial bit of protection just in case.
 
Check the DC offset at the speaker outs before hooking it up to anything, more than about 30mV I’d suggest it needed trimming, but for that you will need a manual to establish what trim pot impacts which voltage and what it should be etc.
 
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Tony L said:
Check the DC offset at the speaker outs before hooking it up to anything, more than about 30mV I’d suggest it needed trimming, but for that you will need a manual to establish what trim pot impacts which voltage and what it should be etc.
Click to expand...
Thanks, the circuit board is really quite simple for this amp, I can only see one pot (on each channel), so hopefully if it's out that'll be the one.
 
Jeddy said:
Thanks, the circuit board is really quite simple for this amp, I can only see one pot (on each channel), so hopefully if it's out that'll be the one.
Click to expand...

That will most likely be the bias, so don’t adjust that unless you know exactly what you are measuring and where to measure it! You need to track down the service manual or find someone who knows the amp.
 
Aha, they are these... (well almost, the ones I have also say Costa Rica) and the pins appear to be where the slots on the side are and these appear to be a 10KOhm trimpot. I'm struggling to find anything about this (old) amp online let alone service manuals.

Is this starting to look like a case of 'twiddling' these to get a value close to 0mv on each channel if needed? (I did check the voltage earlier and way over 30mv, but forgot to 'short' each input channel, so no telling what they were amplifying - or I may be talking rubbish).

(Image removed as off topic)
 
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The point I’m trying to make is the pot is almost certainly the bias voltage, and bias is a very different thing to DC offset at the outputs!

PS I’d recommend starting a thread about the amp, chances are someone here knows how to service it but they may not be looking in here.
 
While looking for something else I've just come across the original JR149 instructions, the tri-fold brochure and a little red cloth (maybe for dusting?). Also a sheet of A4 about the JR150. I'm not sure if similar is already available online, if not and anyone is interested, post here and I'll try and upload a scan. The brochure shows all the original designs for the tops, including what looks like a rather hideous gold top - wonder how many of those were bought...
 
Cool, I’m sure many would be interested. Never heard of the red cloth before!

Which drivers did your amp take out, and what have you used as replacements? As mentioned upthread mine were so obviously mismatched for whatever reason I replaced the lot with the new Falcon B110s and T27s.

PS Instructions as to hosting and linking to images can be found in the FAQ in the reference room.
 
I'll upload those anon then. Thanks for the pointer to the FAQ for instructions.

The amp took out 1 bass unit, I've bought a new pair of Falcon B110s and replaced both. I believe (hope) all units are now ok (two drive units are showing around 6 Ohm and the other two around 6.8 Ohm from memory).
For interest it looks like my amp is a Class AB amp.
The better news is that having left the amp on for a while I'm now getting more reasonable DC offset values of 38mV and 42mV. Maybe not quite ideal, but for an 8 Ohm speaker probably not so far off the mark to be a concern.
 
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