Over Exposed Exposure.

[Welder]

I recently got my Exposure 21Pre and 18 Power back in the system. They’ve had some fairly extensive modifications courtesy of a mate of mine who is pretty red hot on electronics, but possibly not that interested in the end result audio experience.

Overall I’m very pleased with the end result but they are definitely don’t have the sonic signature they did have.
I’ve always loved Exposure kit for their musicality. They have seemed to suit my system pretty much with every model I’ve been fortunate enough to own or try out.
Anyway, on the good side, the combo now has considerably more grunt in the bottom end; tight and well defined bass through the Volts, noticeably better than I remember and an improvement over my 25RC integrated.
Midrange has also improved, better resolution I think is the audiophile term with a greater instrument separation
However, what is causing me a few problems is that from around 1.5Khz to my ears at least, they sound very slightly harsh at times compared to the 25RC.
I could probably mess about with the caps in my speaker crossovers and smooth it out but I wondered if any of the resident cap swappers thought that some changes in the amps output caps might just take that edge off?
Has anyone modded these or similar?

[Reid Malenfant]

What's the bias setting on the output stage of the 18? Might be as simple as tweaking it a tad for minimal crossover distortion...

[Welder]

I don’t know Mark. I’ll try and find out.
I don’t have much in the way of measuring kit here. Hopefully I’ll be able to borrow some kit next week. :eyebrows:

[Reid Malenfant]

All you need is a fairly accurate multimeter to read voltage down to millivolt level, most will do that ;)

Once we know the emitter resistor value we can measure accross both & work out the quiescent current using our mate ohms law :)

Most solid sate stuff is good to at least 1KHz, above that frequency feedback tends to fall with a lot of kit so distortion increases as there is less feedback to correct things...

Lets see what you measure & go from there fella :)

[Welder]

Sound. I must admit amps are a bit out of my comfort zone without fairly explicit instructions.
A straight build isn’t too bad but this fine tuning and fault finding is just one of the reasons a palmed the kit off on my mate in the first place.
I’ll have a tinker at the weekend and get back to you and go from there possibly with your help if necessary (?) :)

[Reid Malenfant]

I'll help as long as you take a pic so i can point you to the emitter resistors John :eyebrows: Get that daughter of yours over with the camera in the phone ;)

There is also likely to be a small preset potentiometer pretty near the output stage that is used to adjust the quiescent current. In a straight class B amp i'd expect about 20 - 25mA bias per output transistor pair (positive & negative), i suspect that the Exposure is likely to be more class A/B & require a slightly higher bias.

[Ali Tait]

John, how long have you been running the amps? They may just need some burn in time.

[Welder]

No trim pots basically.
The 25RC doesn’t have them either, but the Quantum does.

I’ve got a parallel resistor arrangement feeding 3 transistors per channel and it looks like I’ve either got a duff resistor or a high resistance joint.

The DC offset is way out and the current output from one resistor is way down.
Waiting for some new resistors. I’m going to replace all six.

Surprising really that my ears hadn’t identified the problem being just one channel
As I understand it a higher quiescent current within the thermal capabilities of the transistor and heatsink arrangement might yield benefits.
That would mean a bit of messing about with resistor values I imagine.
Need to find out the spec of these trannies really.

Got nice new Panasonic and Phillips caps in there as well I noticed and nice new BCH slit foils on the power supply.
I might replace the trim pots in the Quantum’s to 25 turn sealed units while I’m tinkering.

Hello Ali.
There may be an element of either burn in, or getting used to the different presentation from the 25RC once the amps are set up correctly. In this case though there is definitely something not quite right.
I think once I’ve got the channels matched then given my speakers are set up specifically for the Exposure 25RC, I may find a bit of crossover fine tuning is required.
I can’t complain really, the guy did the rebuild as a bit of a project and all the components were “borrowed” from where he works and the combo does sound rather good even with the current problem.
It’s just I’m a bit fussy.

[Ali Tait]

Aren't we all? :lol:

[Reid Malenfant]

Sounds like it's a good job i suggested you take a look inside ;)

I'm assuming when you say 6 emitter resistors that there are 6 output devices per channel? Or are we talking about drivers here as well?

There should (in fact i'd be amazed if the wasn't) be some other transistor mounted on the heatsink along with the output transistors. It might be a small signal or medium power device, this transistor will be in control of the quiescent current. The only way there won't be a transistor there is if you have some kind of complementary feedback output stage. In which case it'll be on a seperate heatsink along with the drivers or pre drivers (depends on if the output stage is triple or double transistors to give the required current gain).

[Welder]

:lolsign:I have had a look inside.


This is pretty much the layout I’ve got, although it isn’t the right model.
However, no trim pots means no means of altering the quiescent current without some major reworking.
I don’t know all the right terms for the various stages and I haven’t got a spec sheet or a circuit diagram for the unit.

With one good side to match too I should be able to bodge my way to a satisfactory solution :eyebrows:

http://www.mediafire.com/imgbnc.php/928245defdc1782547d605280193fd142233cdc39815010b6a f57ce763949bdb2g.jpg (http://www.mediafire.com/imageview.php?quickkey=ykk26u29j7vb7zj&thumb=5)

Worst come to the worst my mate says he’ll sort it but I’ll have to wait a bit.

[Reid Malenfant]

Hi John, is that picture of half of one channel? Ok, i know it's not the same, but i have never seen a three transistor output stage :eyebrows:

[Welder]

That's one channel mate ;)
I think its the LH

[Reid Malenfant]

That's one channel mate ;)
I think its the LH
:scratch: Bloody weird, unless possibly the centre TO3 is used to set the bias :eyebrows: Never seen such a thing, there are 3 emitter power resistors as well & the positive speaker output disappears off to the left side & out of view - as if off to connect to more power transistors :eyebrows:

That's some weird sandwich you have there ;)

[Welder]

Well, I don’t know nearly enough about amp design to say whether it’s unusual or not but they make a very pleasant sound when they’re sorted :)
It’s nothing like the Quantum for example which has a more common 2 transistor arrangement and as I mentioned trim pots and is a lot easier to understand whats going on.

[Reid Malenfant]

Next time you are inside get the transistor numbers ;) I was thinking it'd be nuts in certain respects to use a TO3 transistor for the Vbe multiplier (quiescent current control), but if it was a darlington it could be a good one as there is plenty of current gain at low currents ;)

I'd love to see a circuit diagram of this thing :eyebrows: It's definately a bit of an oddity...

[Welder]

I found this when I was searching for info.
While it’s not quite the same, the overall layout seems to follow most of this range of Farlow amps.

Might give you an idea of the basic design.

http://www.mediafire.com/imgbnc.php/5b79b8ed854e43d67b43940fe46f800a96e502161fd3cd53e3 07addec71a6a372g.jpg (http://www.mediafire.com/imageview.php?quickkey=eeu73vdafamoq8v&thumb=5)

[Reid Malenfant]

I guess that the centre TO3 transistor in each output most likely is there to control the quiescent current then :scratch: After all i have never in my life seen an amp with two positive output transistors & one negative or vice versa per channel ;) Well not a class B or A/B at any rate,,,

[Welder]

Oh well, best cross my fingers and plug the orifices then :eyebrows:
Apparently I should be looking at around 56 mA per resistor and should be able to achieve a DC offset of around 0.020 mV :scratch:

(think i need to check this coz that seems very low)

[Reid Malenfant]

0.056A or 56mA is not a bad bias in fact ;) Much better than 20 - 25mA on most kit per output transistor pair..

As long as the DC offset is less than 0.05V or 50mV i wouldn't fuss too much, though yes, i'd prefer to see 20mV or so!

[Welder]

Well I’ve got a few resistors on the way and may just see what it sounds like if I give it a bit more current if the trannies can cope.

[Reid Malenfant]

They should do as long as you don't go nuts, TO3 are about as robust as you'll get ;)

I'm not sure of the power rails or the heatsinking but if the rails are say 40V each then 100mA shouldn't be a problem. That's only 8W dissipation per channel.

[Welder]

Cheers for having a look anat anyway Mark ;)
I’ll let you know how I got on.
I should have the bits Tuesday.

[Welder]

Yessssssssss, sorted :)

The problems were, a few high resistance solder joints, one on a transistor leg, another on a resistor.
One small section of track had a crack in it. That was a bit fiddly to sort out as there wasn’t a lot of room at that point.
All emitter resistors replaced and a variation of less than 8 mA over the 6 achieved.
I now have a Dc offset 13 mV left channel and 20 mV, acceptable I think.
I also changed the speaker output banana sockets which weren’t giving the best connection with the plugs I use.

I’m still a bit surprised I could hear the effect of the faults; maybe my hearing isn’t as dire as I had assumed.

They’re sounding excellent now; a noticeable improvement over the 25RC. They still have that “bags of grunt” feeling I tend to associate with Exposure amps but have gained some extra control over its use.
I’m usually a bit stumped when trying to describe differences in what I hear but Dave (DSJR) got it for bass at least. The slightly uncomfortable sensation one gets from a pressure wave in the lower Hz region is more pronounced. It’s not more bass as such and my trousers are definitely not flapping, just a different delivery, more punch, less steam roller. I also seem to have acquired improved percussion decay.
The 21 & 18 combo does give an extra 15 watts of power which may have some bearing depending on how the amp draws availible power I imagine.

I would love to hear an unaltered combo to compare mine to, just to see if there is a difference between them.
Anyway, these will do me until someone gives me a NAD M2 :lolsign:

I think that’s me just about done with Hi Fi. I’ll probably continue to tinker with the music server and possibly make what I have look more appealing but with regard to buying new kit; well, I can’t afford anything that will give what I consider to be worthwhile improvements and some imminent changes in circumstances will probably mean I’ll be spending a lot less time at home and even less in front of the computer.......