World-Designs-Forum  

Go Back   World-Designs-Forum > DIY Projects > Amplifiers
Register FAQ Members List Calendar Gallery Today's Posts Search Today's Posts Mark Forums Read

Amplifiers Your DIY amplifier designs

Reply
 
Thread Tools Display Modes
  #1  
Old 27th January 2012, 10:23 PM
w1nchester32
Guest
 
Posts: n/a
Default how to reduce input sensitivity on Leak St 20?

Hi... first post so please be gentle! I am using a rebuilt Stereo 20 (by Paul at Classique Sounds) with very pleasing results. However, as the amp is extremely sensitive (150mV for full output) it limits my preamp options. I did initially try an Icon Audio passive pre-amp but this lacked sparkle so I am currently using a solid state Naim 32.5 (with HiCap power supply) which sounds wonderful but i am keen to hear other preamps and particularly keen to hear what a tube preamp can do.

So, I have just recently (and very ignorantly) imported a beautiful vintage Audio Research SP3A-1 pre-amp from the USA to use with the Leak but have discovered, with the help of Graham Tricker, that this is probably the worst sensitivity match imaginable as the SP3 preamp has enormously high gain of 34dB!

I have had offers from well known Techs to modify one or the other to make them work together. Before i do this though i wanted to hear advice from more experienced WD folk.

Is it a simple matter to reduce the sensitivity of the Leak to allow it to be used successfully with a range of differnt pre-amps? Does doing this alter the character of the sound? Using those Rothwell 'in-line attenuators' seems to rob the sound of presence and sparkle.

Perhaps it is better to have the SP3 modified instead? Any help you can offer would be gratefully received....thanks...Winch32
Reply With Quote
  #2  
Old 28th January 2012, 09:53 AM
John Caswell John Caswell is offline
Super Moderator
 
Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,487
Default Re: how to reduce input sensitivity on Leak St 20?

Hi Winch,
As you have discovered that is the problem with attenuators, they tend to lose sparkle. You can alleviate this some what by bypassing the top section of the attenuator with a small capacitor, but ideally need an oscilloscope and a square wave generator to see what is going on.
It is a not easy either to modify the Leak as that will tend to mess it up as well.
Not sure of the best way out of this problem, the two items seem totally incompatible.

John
Reply With Quote
  #3  
Old 28th January 2012, 10:39 AM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 4,718
Default Re: how to reduce input sensitivity on Leak St 20?

Morning John, all

Firstly the gain, I think, is 23dB according to http://www.arcdb.ws/SP3/SP3.html so that may make the job viable. I'd look to lose 20dB to keep just a little extra gain.

Output imp is 600R and recommended input is >20K so that's good.

Only real problem is where to put the attenuator. This might depend on whether your Leak has modified inputs. The original ones are very short links to the first valve iirc.

Anyway, ignoring that, I'd put a divider on the Leak input made of 0.6W metal film resistors values either,

18K series with 2K shunt for 20K input impedance or
43K series with 4K7 shunt for 47K input impedance.

The Leak has a 1M input impedance from gridleak R1 which can be ignored (paralled by the divider shunt) or removed.

Perhaps the easiest way would be to swap R1 1M to the chosen shunt of 2K or 4K7, then replace the wire link between input and node of R1/R2 with the chosen series resistor of 18K or 43K respectively.

I've just worked with values from Maplins here and you can then tweak the divider if needed after checking the subjective result.

It's a bit fiddly in there Winch and I'd strongly advise a tech like John to do this for you if you haven't done similar before
Reply With Quote
  #4  
Old 28th January 2012, 06:41 PM
w1nchester32
Guest
 
Posts: n/a
Default Re: how to reduce input sensitivity on Leak St 20?

Thank you Richard, thank you John for coming back so quickly with advice. Whilst i have access to a tech it seems risky (and a bit non-sensical now i think about it) to attempt to modify either of these classic amps to make two clearly incompatible bits of kit work together....

i think i need to find another power amp that would work with the ARC...does anything jump to mind? My speakers are early 80's Heybrook HB3's.

Would the ARC preamp work OK with an existing solid-state Naim NAP 160 i have?

I havent tried this yet due to advice that valve preamps with solid state power amps (especially 'vintage' ones) can be potentially 'problematic' due to the high transients generated in tube preamps and that this particular Audio Research SP3-A-1 preamp has had the 1976 ARC mod whereby the output caps are removed (i.e. 2 x R68 resistors removed, 2 x C47 capacitors removed, 2 x C39 capacitors removed and 2 x C39 capacitors replaced with a jumper wire). This made an improvement in sound, but created 'issues' with power amps with no input caps for DC protection.

Would the Naim NAP 160 fall into this category? Or would it be OK to use it with this pre-amp?

thanks again for your help with this!

Winch

Last edited by w1nchester32; 28th January 2012 at 06:43 PM. Reason: bad grammar !
Reply With Quote
  #5  
Old 29th January 2012, 02:09 PM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 4,718
Default Re: how to reduce input sensitivity on Leak St 20?

Hi Winch,

If there's a question over the pre having dc or ac offset on its output I wouldn't put it with any power amp which didn't have an input blocking cap. Most power amps assume their source will be safe and do not have a cap on their input. I would expect Naim to be similar.

I would say the Leak is compatible - or easy to make so - and will suit your Heybrooks.

20dB reduction is -10x and is easily manageable with a divider. To put into perseptive, a typical volume control works over a 70dB range which is 3000x.

To match the Leak to the pre needs to change its sensitivity -20dB = -10x making its 150mV sensitivity 1.5V. That's the input sensitivity you'd be looking for in another power amp. There may be some but not many as line level is nominally 775mV and many manufacturers had their own anyway. Quad II would be good at 1.2V (would still need a blocking cap adding) but their solid state went to 500mV etc.

So, fit a blocking cap to the output of the pre, or the input of the Leak, and a divider to the input of the Leak. Careful where the blocking cap goes and the cap and divider resistor values you go for as they may make a high pass filter with each other (the divider just needs to be -20dB, a range of values would do that, and a cap value can be chosen to suit roll off).

Or... make a "passive pre" style box located very close to the Leak input containing a blocking cap onto a volume control. This would allow you to set input sensitivity of the Leak exactly to suit. A cap value of 0.22uF onto a 100K pot would give roll off at 7Hz which would be fine.
Reply With Quote
  #6  
Old 29th January 2012, 03:32 PM
davebms davebms is offline
Senior Member
 
Join Date: Nov 2008
Location: ware ,herts
Posts: 205
Cool Re: how to reduce input sensitivity on Leak St 20?

Or... make a "passive pre" style box located very close to the Leak input containing a blocking cap onto a volume control. This would allow you to set input sensitivity of the Leak exactly to suit. A cap value of 0.22uF onto a 100K pot would give roll off at 7Hz which would be fine.[/quote]

w1nchester32. i think the above idea is very good ! i have an ARK pre with to much go ,for my 6550 amp .i use a pot in/box its fine , at least you will be able to use what you got (rather than sell that to bye this)and on-on-on
Reply With Quote
  #7  
Old 30th January 2012, 08:03 PM
Ardee Ardee is offline
Junior Member
 
Join Date: Sep 2006
Location: UK
Posts: 22
Default Re: how to reduce input sensitivity on Leak St 20?

The ideal thing would be a Transformer Volume Control. Second best, a cathode follower based preamp. Reducing the amplification stages in the chain is always going to be better than reducing unwanted gain IMO.
Reply With Quote
  #8  
Old 1st February 2012, 01:12 PM
w1nchester32
Guest
 
Posts: n/a
Default Re: how to reduce input sensitivity on Leak St 20?

Guys, thanks so much for your help and advice....
Reply With Quote
  #9  
Old 18th February 2012, 11:14 PM
bikerhifinut bikerhifinut is offline
Senior Member
 
Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 984
Default Re: how to reduce input sensitivity on Leak St 20?

Hi,
Just happened on this as I've not been online for a good while.
I've used a Stereo20 for a fair few years, it's what got me hooked on Valve amps!
There is a very good Leak forum on Yahoo tech groups, and you'll find a good bit of info. Some there have modded their Stereo20's using ECC81's as a lower gain valve and then tweaked the negative feedback to give the correct overall level.
It's not a trivial thing, although you can keep the same resistor and capacitor values on the phase splitter. Also It wouldnt be Leak anymore and may have a significantly different sound presentation.
The suggestions about attenuators will all work, however I feel that you'd be just as well using a passive volume control.
I have had good results using a Cathode follower buffer with an ECC82. I even got away with using the power outlet from the Leak, but it's far from Ideal as the heater supply is referenced to ground at 0V and you run the risk of heater/cathode insulation breakdown. Theres a way round this but a bit complicated. The HT voltages from the preamp socket may be more suitable for use with an ECC88 as a CF. I'll have a look at the circuits.
Finally I have to confess to a travesty! I usually use a Rega Cursa3 SS preamp with my valve power amps. I find it suits my tastes and its very useful because the gain is programmable from unity up to a level suitable for use with a quadII. If you are a silicon-phobe then I guess its not an option! All the rega cursa variants are suitable as they can have their gain adjusted.

Andy.
Reply With Quote
  #10  
Old 19th February 2012, 10:02 AM
Richard Richard is offline
Super Moderator
 
Join Date: Dec 2005
Location: Notts
Posts: 4,718
Default Re: how to reduce input sensitivity on Leak St 20?

Yes a passive pre works great with ST20, I ran mine that way using an Alps wired as a shunt pot. It may depend on how much Winch wants to use the AR, perhaps if he's wanting the vinyl phono facility.
Reply With Quote
Reply

Tags
audioresearch, leak, sensitivitymismatch

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT +1. The time now is 02:18 PM.


Powered by vBulletin® Version 3.7.3
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright World Designs