Kinshaw Perception Pre Amp needs revamp !

[RCruz]

Having been used to find help in this forum, I decided to open this thread about the Kinshaw Perception range.

I have one power amp in perfect working order but the preamp produces a terrible noise (motorboating).

I measured the preamp psu and it is outputing +-15v.

As I can not find schematics anywhere, I hope someone can help me debuging this one.

Here is the link to the manufacturer site:

Kinshaw - sound without compromise

Best regards

Ricardo

[Mooly]

Are you sure it's motorboating as that is quite a specific low frequency instability.
I have only ever come across one instance of that in solid state gear and it was on a Toshiba TV with TDA3000 (it does exist) outputs. PSU fault was the cause here... common fault at the time.

First thought has to be dried out caps...
A check with a 'scope should pinpoint a fault like that in minutes.

Are both channels affected, if so that points to PSU issue... caps ?

[RCruz]

Both channels are affected.

I do not have a scope.... can I use some kind of resistor ladder to connect the psu directly to a speaker (so I can hear the psu alone )?

[RCruz]

This psu has two 22000u 25v blue Philips caps.

Are these known to fail often ?

[Mooly]

I'm not familiar with the amp at all... but instability and motorboating have definite causes.

Both channels affected points to a PSU problem.
That's inventive using a speaker. You must AC couple the speaker with a largeish cap >220uf and use (at first) a series resistor of say 220 ohms to initially charge the cap and see if you hear anything. Observe correct polarity of cap Once the cap is charged short the resistor out, and a stable PSU should be silent.
Discharge the cap via the resistor each time you move to a new voltage/rail etc.
Try it on the unregulated as well as regulated side. Common 7815 7915 regs can do weird things sometimes, might not show on a DVM... hence the scope recommend.

[RCruz]

It seems quite a complex procedure... I guess I will replace the caps and verify.

Thank you so much !

Ricardo

[Mooly]

Lol... what's complex about that... no sense of adventure

You need to try and pin the fault down, otherwise you could waste time and money.

How old is this thing anyway... is it a classic sort of design... pure analogue audio or is it microprocessor controlled with loads of facilities etc.

[RCruz]

It is an opamp based pre with an opamp based riaa.

Has balanced outputs.

I do not have any spare caps anyway so I guess I will give it a try with the speaker test... Why must I use such a large cap ?

Can I use a 10u film cap instead ?

[Mooly]

A good chance it is just a cap or regulator fault then. Sounds a fairly straightfoward design.
You need a largeish cap to ensure that low frequencies are passed to the speaker. Try your film cap... it won't do any harm, but any LF noise on the PSU will sound faint with only 10uf coupling to the speaker.

You can place caps across any suspect ones to prove if faulty. Even a cheapo 470 or 1000 uf cap of suitable voltage is OK to bridge those 22000 ? uf Philips ones. Don't discount a regulator as being faulty... it happens.

[RCruz]

Thank you so much.

I will keep you informed of my progress

Ricardo