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Have similar issue with my Rotel poweramp.
Hi gain poweramp into a sensitive pre, with efficient speakers...you can hear the noise floor.
Preamp has tubes on the input stage, and tried mitigating it with Different tubes, but may go down the resistor change route myself.
I purchased a Micro 25 basic pre amp new several years ago, and at the time was using a leak st20 power amp, Glenn lowered the output of the Pre amp to match the input sensitivity of the leak.Quote:
Originally Posted by Wegamus
When I replaced the basic with a exdemo standard micro 25 I contacted Glenn and he was happy to provide the information to change a feedback resistor (I believe) per channel. He also offered to complete this if required at a reasonable cost.
Sent from my SM-G950F using Tapatalk
To be fair Dave the issue raised is well articulated and clearly of genuine 'real world' concern. I say this as a loyal and long term Croft owner (purchased from yourself !) I just thought your comment to be a little towards the dismissive side. But yes, I'm sure the issue can be sorted by GC if the valve change doesn't address it.Quote:
Originally Posted by hifi_dave
Sorry to appear dismissive but as someone who has probably sold more Croft than anyone since the early 80s, this has never been an issue. What happens though, is that it gets mentioned on a forum somewhere and then becomes 'fact' and gets spread around, so we end up with these great products being run into the ground.
Occasionally, when a customer uses a Croft pre with another amp, such as a Quad, I order the pre with lower gain and then there is no problem.
As to the 'hiss' in this case, it is most likely a noisy valve, which Croft will replace FOC. In this case, the amp is s/hand and we have no way of knowing what has happened to it.I have never, in the 12 years or so that this latest Croft range been around, had a fault of any description and there isn't much you can say that of.
In a similar vein, one of the posters here had a Rega amplifier and he phoned to complain about a 'hum' on the left channel. In discussions it turned out that he hadn't actually experienced the 'hum' himself but had read about it on forums. The major 'hum' he could hear was when the amp was switched to phono, turned up to full volume and with ear to speaker, a 'hum' could just about be detected. Is that a real problem ?
It does make you wonder about the gullibility of some folks..Quote:
Originally Posted by hifi_dave
My 12BH7A should be here in a couple of days. Which one do I need to swap it for? There are two ECC83S and an unknown Tesla one that’s a bit smaller than the other two:
I have my spare Croft Integrated here so could try swapping over an ECC83 to see if it was a faulty valve.
https://i.imgur.com/HCa9Cgy.jpg
Emailed Glenn and he told me which one to swap. Did the swap over...and the hiss is still there, so doesn’t seem to be a faulty valve.
Have to say I’m very much enjoying the music with the 25R/7R and can see me keeping them long term, which is why I’m trying to sort out this niggle.
It's the valve that's on its own, the closer one in the above picture.
What have you changed the ECC83 for?
Do you get hiss with the volume pots on min (sorry you may have already said)?
If you fit shorting plugs to the 7R inputs instead of the connection to the 25R do you still get hiss?
Yep, I confirmed which is the audio valve with Glenn. Thanks though. I just switched the 7R ECC83 for the same valve from my Croft Integrated. I’ve got two NOS 12AT7WB here but I don’t think they are suitable.Quote:
Originally Posted by Firebottle
The hiss is independent of the volume position of the preamp. Once I turn the preamp volume up near the top of its range there’s a slight increase in hiss but that’s to be expected.
Don’t have any shorting plugs here, used to have some Russ Andrews Shorties but no idea where they are now.
The 12AT7 is a lower gain valve and won't cause any damage if you try it. I suggest you do.
You’re sure it’ll work ok?Quote:
Originally Posted by Firebottle
They are GE JAN 12AT7WB.
Yes absolutely. The 12AT7 is equivalent to ECC81.
How long should I wait for the amp to cool down before removing a valve, is it dangerous for a long time after switch-off?Quote:
Originally Posted by Firebottle
Right, I’ve changed the 7R audio valve from the ECC83 to a ECC81. Don’t think there’s any difference. Still hissing like before.
OK. Power the 7R without anything connected to the inputs.
You may get a sort of hum from the input not being connected to a low impedance source, but listen and see if there is hiss.
Right, I’ll try that now...Quote:
Originally Posted by Firebottle
Should I put the ECC83 back in first? How long should I wait to do that if so?
I got the hum like you said, but hiss was much less than before and was a bit hard to hear through the hum. There was minimal hiss.Quote:
Originally Posted by Firebottle
It’s worth noting that the guy who sold me this amp told me it didn’t hiss at all when directly connected to his DAC with the DAC as volume control. The 25R he had didn’t have any hiss with another power amp he tried either. But when used together the 25R/7R had hiss, just like I’m finding. So individually the Crofts were fine, together they hiss.
There is another option with Croft gear, one that Glenn offers.
The output/line stage of the 25R (and other models) can be converted to a zero gain buffer (cathode follower). This removes the gain given by this stage normally and will allow further travel on the volume pots.
I have modified a Croft pre into this configuration previously.
But will that stop the hiss? Is it the preamp gain that’s likely causing it?Quote:
Originally Posted by Firebottle
I believe it will and yes the gain is too high.
Oh great, sounds like we’ve identified the cause and solution of the problem. If I were to have the preamp mod you mention, would I also be able to comfortably use a Quad 3O3 with the preamp too? I note that both the 7R and 3O3 have 0.5V sensitivity but I don’t know if any other factors are relevant.Quote:
Originally Posted by Firebottle
I’ve had the 12BH7A valve fitted to the power amp input for a few days now and it’s a lot better in terms of hiss and there’s a bit more range on the volume control too. Still not as quiet as my NAD integrated but quiet enough to not really matter anymore.
:D:)