Pre3 into KAT6550 anyone?

[RobHolt]

About to order a Pre3 kit for use with my KAT6550 and was wondering if anyone has used this pairing. IIRC the gain on the Pre can be adjusted internally and was wondering is anyone could confirm this?
I've previously run a Pre2 and been happy apart from some hum issues which I understand are less likely with the transformers in the Pre3.

Also, what do folks advise as a suitable input impedance on the KAT6550 (its currently a KIT6550)?

Will be driving 5m interconnects.

Ta

[John Caswell]

Hi Rob,
Not much you can do about the input impedance of the KIT/KAT 6550.
What I suggest is the you remove all input and tape switching on the KAT and any unwanted phone sockets and wire one pair of phono sockets directly to the inputs grids of the ECF80s via, say, a 1K series grid stopper (if not already there) and of course the grid resistor to ground.
The PreIII overall gain can be reduced to half (-6dB) by using the shunt volume control modification with a 51K series R for the top half of the volume control or less.
If you wish to drop the "gain" even further there is a wire link on the PCB (LK1 I think) that couples the first anode to the second grid of the ECC82. This link could be replaced with say a 470K resistor and the following grid resistor reduced to 470K to give you another -6bB, this is a classic "L" pad attenuator or just juggle the two values to adjust the gain to your liking keeping them both in the hundreds of K region. Don't attenuate the PreIII output or similarly the 6550 input as this will tend to destroy the general dynamics of the setup as the PreIII out[ut impedance is very low (10R).

John - Dr John

[Richard]

Hi Rob,

Mine and a friends are used with 300mV 100k WAD valve power amps, great sound and no problems at all.

First thing is to assure you Pre3 is stoney quiet. The only noise we encountered was with noisey valves and you'll recognise that by the low level "fluttery" sound they make on one channel.

Used with Phono3 and 3S it is also very quiet, just a small rise in thermal noise with an ear by the speakers and quieter than many ss designs. The specs back this up and seem quite achievable. My only reservation is that with the step-up TXs installed MC phono then becomes susceptible to hum depending on placement. I removed mine from the case and will install them externally in a remote heavy iron box if they are needed in future.

Prepare Kit6550 by removing the inputs, selector and volume pot and take the input directly to the junction of R7/8 with R9/10. Replace R7/8 1M with 100k. That gives you a line sensitive 500mV power amp with 100k input impedance (Kat6550).

Pre3 has 6x gain (+15.5dB) and approx 12 ohms op impedance so will drive just about anything. You won't need all that. The best way to change it is at the volume control. Go to the bottom of the Shunt Pot faq and choose the 47k resistor with 20k pot option. This is what we use, leaves Pre3 with x2 gain (+5dB), and works well into line level. The values chosen keep Pre3 input impedance around 50k and you'll see it doesn't change much with level (not that sources exhibit change at these high impedances anyway).

Rich

[Richard]

Hi John, all,

Following up on your suggestion for lowering the gain of Pre3.

Is this correct, with values of the 2 resistors variable in the divider? Could a 1M pot be used there? Are there any downsides and have you used it successfully?

Many thanks.

[John Caswell]

Hi all,
Thanks Richard
No not 470R - 470K please and yes you could substitute a 1 meg pot just as well, set the gain, and then measure the two legs of the pot and replace it with fixed resistors. Also agree with Richards comments re noisy ECC82s you need to select quiet ones

John - Dr John

[Richard]

Thanks John R changed to k !

[RobHolt]

John, Richard,

Many thanks for the responses - will give it a go.

regards,

Rob.

[RobHolt]

Guys, another quickie while I'm here.

As I mentioned on another thread I recently dusted off and recapped the 6550 as it was humming/vibrating. That's now sorted, however I have always had an intermittent hum issue with the amp which manifests as follows - wiring on the inputs is as per the build manual:

If i switch the amp on with with no sources connected I hear nothing from the speakers apart from a gentle hiss. No hum at all which is fantastic.
If I then connect one channel of any source, still I get no noise.
If I connect both channels I get hum. It's not loud but audible from the listening position.

If I disconnect the input earth running from either channels common phono input sockets to the volume pot the hum almost disappears and the amp still plays fine.

I realise these earth loops are difficult to diagnose at a distance but any idea would be welcomed.

PS: disconnecting the mains earth temporarily has no effect.

[John Caswell]

Hi Rob,
Yes you have a classic input earth loop. You need to check how the earths are arranged on the amplifier, I suggest after you change it to single input. Usually it is due to the input earths being "commoned" so you may have to separate them totally from input socket to input grid to earth returns.
For the time being however you can just lift one of the cable screens.

John - Dr John

[Richard]

Rob, a couple of ideas,

We've had this before with double insulated (2 core mains) cdps and record decks like Thorens and Rega which use a phono earth braid as their earth connection.

With a record deck it's usually best to break the connection inside the deck and use a separate wire between deck metalwork and amp earth post.

With a cdp this may not be so easy.

1) Try commoning the earths at the amp end so the eddy current doesn't pass pass across the channels in the circuitry. Many amps have the earth's commoned at the phono tags or shared earth point where they connect to the pcb. With your amp the paths are separate I think. Easiest way to try this is simply to connect/hold a piece of wire across the external outer shells of any 2 L and R channel phono sockets and listen for a change.

2) I cannot see an earth lift resistor fitted. This may end up being the only way to stop currents circulating through the circuit ground. 3W ww 10R can go between SIGNAL ground wire and chassis earth post. It's usual to parallel this with a high voltage 0.1uF film cap to provide an rf leak. Important that mains earth and mains TX primary earth go solidly to the chassis earth post. We just want to "float" the signal circuitry inside the earthed chassis case if that makes sense. All other connections remain the same and you should be able to verify by probing chassis to various signal ground points (on the tag strips, phono shells, and speaker black post etc) and finding 10R.

Rich