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.

[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 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?

[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).