klpp1+k5881mkii noise reduction?

[jdrake]

hello,
this is doubtless an old one so sorry to rehash, but web search unfruitful.

I'm feeding klpp1 into k5881mkii. The noise level is acceptable for most music
and listening levels, but for very quiet bits hiss can be audible. I suspect k5881 is
rather sensitive for the output of klpp1 (hiss level essentially constant with volume; absent when klpp1 disconnected). If so, any suggestions as to optimum mod for this combination to attenuate sensitivity? Or perhaps there is something defective in my build that might lead to raise noise floor?

Am driving speakers with ~88dB sensitivity (Newform R630) and get plenty out of current combination, so could I think lose a little sensitivity without running into power problems.

Any insights appreciated.
cheers,
Jeremy

[John Caswell]

Hi Jeremy,
The sensitivity of the 5881MkII is fairly high at 75mV for 20 watts out so I sugest an attenuator of some sort on its input. As its input impedance is roughly 100K you can place a 100K resistor from the input socket to the junction of the 1K and 100K on the grid on the first valve the 7025 (after removing the wire in situ). This will give you a drop in sensitivity to 150mV. Putting a 220k resistor instead will drop it even further to 225mV and a 330K will take it down to 300mV.
Another method is to obtain a couple of 500K preset pots and wire them directly across the input and earth with the wiper taken to the input wire and adjust for best level with the KLPP1 volume control set to 11o/c.

John aka Dr John

[NealG]

As John says, 75mV is very sensitive. I used this combo for a while and added a PD as John suggests to give -6db attenuation. I also modded KLPP1, it gives a x3 output on the last stage, you can change R17 the feedback res. to lower the gain. I wired a second 560K res in parallel with it to lower gain to 1.5x.

also are you using LP, say with a MC? Do you have step-up transformers? I found KLPP1 is a little noisy with low output carts fed directly into it...

[Richard]

I can't see that lowering the sensitivity is going to lower the hiss at the same playing volume.

Rich

[A Stuart]

I think the reference was to lowering the sensitivity of the power amp (not the phono amp), so needing to send a larger signal while sending the same level of hiss from the pre-amp.

I had the same problem. The output stage of the klpp1 is noisier than the phono stages! I took to driving the power amp directly from the volume control wiper. NB This loses the low frequency filter function, but I have never found that a problem.

Alastair

[Richard]

Originally Posted by A Stuart

I think the reference was to lowering the sensitivity of the power amp (not the phono amp), so needing to send a larger signal while sending the same level of hiss from the pre-amp.

Thanks, yes, I understood John was describing attenuating the power amp but I was thinking the volume control was on the power amp rather than before the preamp, my mistake, should do the trick.

Rich

[jdrake]

Thanks very much for the tips!

Yes, as Alastair says, I think the dominant hiss (in mine at least) must be from the final stage of klpp1 as it appears independent of volume pot vr1.
There is a hum contribution that rises with volume, but still acceptably low.
Dominant hum appears to be from k5881 - not so bad as to be a problem but if anyone managed to significantly reduced this I'd also be interested to hear the trick.

Phono seems about as quiet as line. I'm putting an early 90's Benz Micro Glider medium output (about 0.8mV I think) straight in.

While fine for quite high volumes I suspect the Newform R630's are a
dB or so insensitive for the k5881. Cranking klpp1 past 2-3 o'clock to
indecent volumes can produce noticeable distortion not present with other amps at equivalent loudness. Can this be caused by overload of k5881 input?
If anyone is looking to sell a k5881mkII I'd be interested for monoblock.

It'll be a couple of weeks before I can try John's or Neal's fixes.
Meanwhile, if by any chance anyone has any info of cost/merit ratio
of US "tube" tuners such as McIntosh MR65b,71, Fisher FM100b etc
would be grateful for any purchase thoughts!

Cheers,
Jeremy

[John Caswell]

Hi Jeremy,
Re the valve FM tuners have a look at this web site "fmtunerinfo.com/" very helpful, but bear in mind a few of things.
Most will be 110Vac
Most will be old and possibly botched unless you are very lucky
They may use valves not easily available in the UK if at all or b****y expensive.
Unless carefully looked after they will probably require realignment, which is a skilled time consuming job.
Apart from that go have a look

John aka Dr John

[A Stuart]

As an afterthought to what I wrote above: a spin-off benefit of
bypassing the output stage of the klpp1 (straight from the volume control to power amp) is that unless listening to records it need not be switched on .

I have not heard (cloth ears?) any ill effects through capacitative loading by cables of phono section nor any other source. However I may be correct in thinking that a valve tuner, which you mention, would need an especially high impedance load.

Alastair.

[jdrake]

OK, thanks. Am actually living in tube land and uk equipment is often
tricky to consider for same reasons. Somewhat of an impulse but have
just committed to a Fisher FM100B c.1965.
cheers, Jeremy