Amp advice sought

[Shadowbox81]

Hi

I'm hoping someone might be able to advise on the following problem I'm having.

I've recently taken delivery of a Croft 7 power amp and having hooked it up to my Forte Audio Model 2 preamp, I'm finding that I'm having to run the volume on the pre at pretty much close to it's minimum when listening to CDs (using a Cambridge Audio D100) otherwise the output is too loud. The situation in a little better when using my RP3 (running RuneAudio) via a Dragonfly Red but I've always found that to run a little quieter than the CDP. With the CDP and the min on the preamp vol being 7 o clock, I advance to 8 or 8:30 before it's too loud.

I'm enjoying what I'm hearing from the Croft so far but could do with a bit more volume control. Having scoped out the issue on the net, I've identified a few possible solutions and would appreciate some opinions as to whether they might solve the issue or indeed any alternatives:

1. Put a set of in line attenuators between either the CDP and pre or between the pre and power - Reports on the use of these appear to be somewhat mixed.
2. Change the stock tube in the croft for a lower gain option.
3. The Forte has a couple of trim pots inside - I could adjust these downwards but am wary of doing so.

Any thoughts appreciated?

[struth]

nice pre.. are you using rca's

[Shadowbox81]

Thanks. RCA are being used on both cd to pre and pre to power sets of ICs

[Haselsh1]

Yes Neil, it is a very common problem with Croft gear and the Croft gear I have also owned. I used 14db attenuators, Golden Jacks type and these improved things quite a lot. Stick with it though, well worth all of the hassle.

[Haselsh1]

Oh yes, I also changed the valve type for lower gain and it just wasn't a big enough drop.

[JimC]

Have you contacted Glenn himself?
I have always found him to be really friendly and helpful and he will know what to advise.

Cheers,

Jim.

[Shadowbox81]

Thanks for the suggestions guys

Shaun - where did you place the attenuators and have you noticed any difference in the sound? Interesting about the valve not dropping the gain enough. What type did you try?

Jim - Funnily enough, contacting Glenn was going to be option 4. I haven't contacted him as yet but may well do so as suggested

[hifi_dave]

Thanks for the suggestions guys

Shaun - where did you place the attenuators and have you noticed any difference in the sound? Interesting about the valve not dropping the gain enough. What type did you try?

Jim - Funnily enough, contacting Glenn was going to be option 4. I haven't contacted him as yet but may well do so as suggested

Rothwell on this forum, make a very nice range of attenuators suitable for this application and no, you won't hear them.

[walpurgis]

I've used attenuators in the past. Whilst their effect on the sound is very minor, it is there and I could hear it.

[IHP]

From the tubestore website...

The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain.

I use a JAN GE 5751 though not for reasons related to gain, just think it's a fine tube. But as above, GC is your chap to help here.

[Shadowbox81]

Thanks Dave/Geoff, it seems like your views pretty much sum up the body of opinion out there. I suppose the only way to find out would be to get a pair and have a listen.

Thanks Ian - I see that JJ valves also do a mid gain version of the Ecc83. I might consider that or the 5751 and see what happens.

On the other option of the internal trim pots, as I said, I am reluctant to mess with them. If I do adjust them however, could that cause any damage to the pre or power amps.

[dave2010]

From the tubestore website...

The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70. This means that if you plug a 5751 into a socket that expects a 12AX7, the pre-amp will have about 30% less gain.

I use a JAN GE 5751 though not for reasons related to gain, just think it's a fine tube. But as above, GC is your chap to help here.
if the gain ratio is 70/100 that represents a dB attenuation of about 10 log (0.7) or about 1.5 dB relative to the tube with the higher gain. There might be a difference if the ratio is power or voltage, but either way the likely attenuation is only going to be around or under 3 dB - barely audible to most of us.

[Firebottle]

From the tubestore website...

[I]The gain factor of a tube simply measures how much it amplifies the input signal. For example, the common 12AX7 tube has a gain factor of 100, while a 5751 tube (which is often used in place of a 12AX7) has a gain factor of 70.

Be aware that unless special circuit techniques are used the actual gain achieved in a typical valve stage is only about 2/3 of the published figure for mu (gain).

So for an ECC83 stage the gain will typically be about x 60.

:)

[Shadowbox81]

Thanks Dave/Alan, that is consistent with Shaun's comment earlier in the thread.

I'll drop Glenn a line but as specs on the Forte are hard to come by, I'll have to see what he suggests. In the meantime, I might give a pair of attenuators a go

[walpurgis]

The Forte is a lovely pre-amp. I had one.

It was a while ago, but I seem to recollect the trimmers were for the phono stage and had nothing to do with line output.

[Shadowbox81]

I think you're right Geoff. Looking at the boards, the trim pots are directly next to the LT1028 opamps with another set of opamps over the other side of the board. So it looks like the trim pots are indeed part of the phono stage section of the circuit. However, my Lenco is currently in pieces as it's in the middle of a rebuild and replinthing so it'll be a couple of weeks before I can hook it up to test that theory.

[walpurgis]

Yes. I think they are for MM impedance matching.

[Shadowbox81]

Interesting. Looks like I'm going to have to do some reading up on that subject before I get the TT back in the game