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Old 29th July 2010, 06:44 AM   #571
barossi is offline barossi  Germany
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Hi,

i could not get rid of my hum, probably produced by my heater supply.

2*16V AC heater trannies, followed by MUR820 diodes and 15000uF then a reg circuit with L200, 15mH choke and then a LT1084 board:

Click the image to open in full size.

And here is a photo of my 26 signal board:

Click the image to open in full size.

What should i do?

KR Barossi
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Old 29th July 2010, 07:12 AM   #572
mach1 is offline mach1  Australia
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If the hum is from your filament supply, you could try adding a common mode choke.

I made a Ronan Reg based filament supply (2 voltage regs followed by a current reg) for a 1LE3 linestage and the amount of cr** that got through was incredible. A change of filament transformer and addition of a CMC made quite a remarkable difference.

However, all that exposed unshielded wiring has got me wondering whether the primary cause may lie elsewhere.
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Old 29th July 2010, 04:52 PM   #573
kevinkr is offline kevinkr  United States
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The transformers are susceptible to local ac magnetic fields, and the 26 is extremely susceptible to electro-static pick up. (Anything closer than 1 meter may interfere) Putting everything in a metal chassis or even just use some copper or AL foil to shield around (under) the circuitry may help. The foil must be grounded to your audio ground to be effective.
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Old 30th July 2010, 01:04 PM   #574
CFT is offline CFT  United States
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I am converting mine to a parafeed output, what would be a good parafeed cap for that? And the value? Thanks for the advice!
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Old 31st July 2010, 02:15 AM   #575
mach1 is offline mach1  Australia
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The optimum value will vary depending on the inductances involved. Try starting with a Russian KBG 4uF 200V. They are a very good quality cap well suited to this function which you can pick up cheaply on ebay.
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Old 31st July 2010, 04:52 AM   #576
CFT is offline CFT  United States
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Thanks Mach1.

I have some 2UF I bought for the T-amp inputs....will certainly give them a try!
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Old 2nd August 2010, 03:41 AM   #577
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Quote:
Originally Posted by barossi View Post
Hi,

i could not get rid of my hum, probably produced by my heater supply.

2*16V AC heater trannies, followed by MUR820 diodes and 15000uF then a reg circuit with L200, 15mH choke and then a LT1084 board:

Click the image to open in full size.

And here is a photo of my 26 signal board:

Click the image to open in full size.

What should i do?

KR Barossi
Most of the hum issues got resolved in my 26 pre after correctly configuring the grounding. It is worth to check.
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Old 5th August 2010, 10:26 AM   #578
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Anyone could an idea how to improve bass response with the 26 pre? (or rather explain ways to fine tune bass.)
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Old 5th August 2010, 03:45 PM   #579
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by coolzero View Post
Anyone could an idea how to improve bass response with the 26 pre? (or rather explain ways to fine tune bass.)
Possibly assuming that other system issues are not responsible it may be time for a really good quality plate to line transformer..No one who has built one of the several versions of my transformer coupled line stage has ever complained about bass response. The low source impedance so provided will allow you to drive any power amplifier well - at the expense of gain of course.

Otherwise you should post your current design and let us have a look.. And tell us what is being used with it in as much detail as is reasonably possible..

An RC or CCS loaded 26 will have a source impedance of about 8K which is not particularly good for driving a solid state amp, nor any amp with an input impedance of less than 75K.. The size of the coupling capacitor plays a role as does any HPF that may be present at the input of your amp, which IIRC is solid state..

Other factors may play a role here, do you have another pre-amp that performs better in this regard? Speaker design, placement, listening position, and many other things may contribute here. What are your speakers - and do they need/use BSC (baffle step compensation)?
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Old 5th August 2010, 05:42 PM   #580
zobsky is offline zobsky  India
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Quote:
Originally Posted by kevinkr View Post
Possibly assuming that other system issues are not responsible it may be time for a really good quality plate to line transformer..No one who has built one of the several versions of my transformer coupled line stage has ever complained about bass response. The low source impedance so provided will allow you to drive any power amplifier well - at the expense of gain of course.

Otherwise you should post your current design and let us have a look.. And tell us what is being used with it in as much detail as is reasonably possible..

An RC or CCS loaded 26 will have a source impedance of about 8K which is not particularly good for driving a solid state amp, nor any amp with an input impedance of less than 75K.. The size of the coupling capacitor plays a role as does any HPF that may be present at the input of your amp, which IIRC is solid state..

Other factors may play a role here, do you have another pre-amp that performs better in this regard? Speaker design, placement, listening position, and many other things may contribute here. What are your speakers - and do they need/use BSC (baffle step compensation)?
Perhaps parafeed output (CCS + step down ungapped transformer), .. at the expense of some gain ...
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