Valve pre amp

ATC & other makes of Hi-Fi equipment
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Valve pre amp

Postby haken8 » Thu Apr 17, 2014 1:19 pm

I am going to try a valve pre amp into my ATC SCM ASLs. I have not read anything on the forum using valve pre amps so I would be interested in anyones experience or views on the subject.
Just for information I am getting a specially built Croft Micro 25 pre amp to drive my 50s. The owner Glenn Croft ( a pleasure to communicate with) has upgraded the wiring/valve tube from the normal spec so that it can efficiently drive the speakers.
Its something I have always wanted to try and I am more than happy to feed back my views on what happens etc. if anyones interested.

Tom

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Re: Valve pre amp

Postby mamo » Thu Apr 17, 2014 3:32 pm

Hi Tom,
I am using a valve pre with my active 50's.
The biggest issue is getting a good impedance match between the pre output and speaker input, there are a couple of discussions and advise on this topic.
I currently use a modwright LS36.5 valve pre which is excellent to my ears, great controlled bass, it has a 10k input impedance, I think the ATC Actives require a 100k input impedance to the 10 / rule is achieved.
Please correct me if I am wrong.
Others to consider are EAR valve Pre's Manley and YBA to name a few, a valve pre does do something special when matched with Active ATC's.

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Re: Valve pre amp

Postby haken8 » Mon Apr 21, 2014 7:31 pm

Hi Mamo
Thanks for the reply and advice. I am getting a Croft Micro25 line stage only which has one valve. I believe it is a JJ Tesla ECC99. I should get it Tuesday.
I have already bought a GE 12BH7 tube to try in it after everything has settled down after a couple of months. Have you tried any different tubes?
Glad you are enjoying the sound of a tube pre amp driving the 50s.
If there is anything else you think would be useful to me I would be grateful.
regards
Tom

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Re: Valve pre amp

Postby Shadorne » Tue Apr 22, 2014 2:26 am

The 10:1 rule of thumb ratio is for input impedance of successive stage to output impedance of previous stage.

So if ATC are more than 10K input impedance then all you need is to make sure the preamp has an output impedance of less than 1k ohm.

If you follow this rule then the frequency response of the preamp will remain as the designer intended. If you do not follow this rule then the frequency response may become modified by the voltage drop across the output stage of the preamp (which will vary by frequency).

Usually this is a bigger problem for valve power amps and passive speakers - making many combinations of speaker and valve power amp a poor choice.
"ATC. Always The Champion. I will not record without them. The best studio monitors. Period." ~ Lenny Kravitz

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Re: Valve pre amp

Postby haken8 » Wed Apr 30, 2014 9:11 am

Anyone have any experience/ideas regarding using an RCA/XLR adaptor for the ATC active speaker to pre amp out? I am using one and getting a buzz through the speakers. I have gone through all the elimination processes and its when I take the adaptors off the female XLR plug that the buzz stops. What I would like to know is how to get around the problem.

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Re: Valve pre amp

Postby MattSPL » Sat May 03, 2014 12:56 am

I used Cardas xlr to rca adapters with ATC scm50asl's and had no problems at all. I also used them with a Krell power amp.
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Re: Valve pre amp

Postby haken8 » Mon May 05, 2014 5:40 pm

Right I have looked into my problem more closely.
There is a buzz coming from the left speaker which does not respond to the volume control on the pre amp ie it doesn't get louder or quieter. The right speaker is OK.
I switched the cables at the output of the pre amp - Buzz still in left speaker. right speaker OK
I turned off pre amp - No buzz from left speaker or right speaker
I disconnected the source (CD) connection to pre amp. Still buzz from left speaker.
I changed over the XLR/RCA adaptors - Still buzz from left hand speaker.
I then re-routed the left speaker interconnect well away from the power supplies at the rear in case it was causing interference - Still buzz from left hand speaker.
I then used a new set of XLR/RCA adaptors in case the first set were faulty - Still buzz from left speaker.
Can anyone think of anything else I can do to isolate the problem? When I re=connect the old pre amp there is no buzz from either speaker. Any ideas appreciated

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Re: Valve pre amp

Postby MattSPL » Tue May 06, 2014 12:01 pm

Sounds like a problem in the pre amp possibly?
Sony BDP-S790 > Matrix Quattro Dac > Digidesign RM2's with SB Acoustics drivers > DIY Dayton Audio UM18-22 Sub

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Re: Valve pre amp

Postby haken8 » Sun May 25, 2014 10:32 am

Well I never managed to solve the problem of the buzz in the left speaker so I had to send the Croft 25 Micro back and get a full refund. i am however really sold on the SQ of Tubed Pre Amps with the active 50s. I suspect the problem was down to the Croft as I swapped everything else around with no problems or buzzes.
I am now awaiting the arrival of a Eastern Audio Avant Pre Amp to try which was heavily recommended by Gary at Audio Emotion ( which is very convincing in my book) He states that many other Active ATC users have used this Pre Amp very successfully and that it is a giant killer for the price. I am looking forward to testing this soon. I will let you all know what happens.

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Re: Valve pre amp

Postby MattSPL » Sun May 25, 2014 9:18 pm

Thanks for the update, interested to hear how you get on :)

Cheers
Matt
Sony BDP-S790 > Matrix Quattro Dac > Digidesign RM2's with SB Acoustics drivers > DIY Dayton Audio UM18-22 Sub

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Re: Valve pre amp

Postby haken8 » Thu Jun 05, 2014 10:42 am

Hi All

Well I received the Eastern Audio Pre Amp and.... There was still a buzz from left hand speaker. I decided to ring ATC and they told me to plug the mains lead for the left speaker into the same output as the reat of my system. In my case a mains conditioned. This sorted the problem although I now have an extension cable trailing across to the left speaker. When I rang ATC again they said it was a mains loop causing the interference. I asked what was causing it and they said anything in the system but that because the ATC Pre Amp (which I had previously in the system) was superbly built there was no interference with it in the system.

Anyway all sorted now.

The Avant Pre Amp is settling in very nicely and is starting to show a good sound quality. I am after a less clinical presentation and it is certainly producing that. When everything has settled I will be tube rolling to replace the stock Chinese tubes. I have 2 x GE 5751s and 4 x Phillips 12AU7s to put in soon.

regards

Tom

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Re: Valve pre amp

Postby MattSPL » Thu Jun 05, 2014 10:47 pm

haken8 wrote:Hi All

Well I received the Eastern Audio Pre Amp and.... There was still a buzz from left hand speaker. I decided to ring ATC and they told me to plug the mains lead for the left speaker into the same output as the reat of my system. In my case a mains conditioned. This sorted the problem although I now have an extension cable trailing across to the left speaker. When I rang ATC again they said it was a mains loop causing the interference. I asked what was causing it and they said anything in the system but that because the ATC Pre Amp (which I had previously in the system) was superbly built there was no interference with it in the system.

Anyway all sorted now.

The Avant Pre Amp is settling in very nicely and is starting to show a good sound quality. I am after a less clinical presentation and it is certainly producing that. When everything has settled I will be tube rolling to replace the stock Chinese tubes. I have 2 x GE 5751s and 4 x Phillips 12AU7s to put in soon.

regards

Tom


Hi Tom

Good to hear you got it sorted :D Now you can sit back and enjoy the Avant and the rest of you system.

Cheers
Matt
Sony BDP-S790 > Matrix Quattro Dac > Digidesign RM2's with SB Acoustics drivers > DIY Dayton Audio UM18-22 Sub

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Re: Valve pre amp

Postby haken8 » Sat Jun 07, 2014 11:34 am

Thanks Matt. The Pre Amp was the one part of my system that needed upgrading so I wanted to make sure I was moving in the right direction. Really the Avant is a steal at the price (£1595) and I managed to get an ex demo one for £1295.
From the first month of settling in its been a very positive period. I was really expecting to pay in the region of £3k - £5k to improve everything. I even questioned Gary at Audio Emotion as to whether this was too cheap a pre amp and he stated it was a giant killer for the price and yes I would have to pay £3k - £5k to beat it. Clincher for me was that he has it in his own system for use with the Active 50s.

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Re: Valve pre amp

Postby MattSPL » Sat Jun 07, 2014 8:25 pm

It sounds like you got a good buy with the pre amp then, especially if it outperforms much more expensive equipment :D
Sony BDP-S790 > Matrix Quattro Dac > Digidesign RM2's with SB Acoustics drivers > DIY Dayton Audio UM18-22 Sub

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Re: Valve pre amp

Postby oktapod » Thu Aug 07, 2014 3:49 pm

I'm with you on the EE Avant with ATCs. I use this with my 100s, with no problems at all. I do find the gain a little high, however, even with the adjustable gain on the preamp. I tried XLR attenuators, but these seemed to rob the ATCs of bass, which was instantly restored when they were removed, so I just live with the slightly-too-high gain.

FWIW I am running all NOS Mullards in mine, though I think it sounds very good indeed in stock form.

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