Wierd Interference on Phone Pre - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th February 2010, 03:35 PM   #1
diyAudio Member
 
JammyBStard's Avatar
 
Join Date: Feb 2008
Location: York, UK
Default Wierd Interference on Phone Pre

You may have seen my earlier posts, I just built this Battery Pacific phono stage, based on the circuit in this diyaudio thread,
Ultrasimple mm/mc RIAA preamp 2
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

I pretty much stuck to this circuit apart from some minor resistor value differences and the coupling cap before the second stage is a 560nf but I dont think that would cause this problem.
I have previously built essentially the same circuit with bog-standard film caps and wirewound resistors and was so impressed I wanted to build it again with decent PP caps and carbon resistors and this time Battery power.

When it is connected to a stylus there is a low level whitish noise like a badly tuned radio and when there is no stylus connected it acts like a microphone, if you tap on the metal case you hear a sound like a metal case been tapped if you cratch the metal caseyou hear a sound like a metal case been scratched. When you play a record it sounds great, but the white noise is still present.

I'm not great with the theory but I always get a great response here; Anyone got any idea what's going on?
  Reply With Quote
Old 9th February 2010, 05:14 PM   #2
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
this is typical, because there are low currents and high impedances. By MM RIAA head amps with tubes the same effects are to observe.
You can reduce the microfonic effects by the use of a very heavy board for putting up your RIAA device (at best sandwich structure, inside a mixture of lead shut and sand for birdcages).

How much is the noise? More or few as that one from the neutral groove from your record disc?

Noise like a badly tuned radio indicates oscillation (an introduce of an gate stopper resistor of 1-5 K-Ohm near at the Gate of the input jFET can help)

If by tap on the metal case I hear a sound like a metal case, it is necessary, to coated your plates with bitumix or similare suited damper stuff (the ventilation and thermal management shall not be influenced !!)

For the ultimate sonic quality microphonic must be greatly reduced.

Therefore, a certain sound quality level isn't break with tube technology. And therefore tube technology by RIAA head amps is only recommend by low and medium cost devices and not for top class products.

Last edited by tiefbassuebertr; 9th February 2010 at 05:29 PM.
  Reply With Quote
Old 9th February 2010, 05:33 PM   #3
diyAudio Member
 
JammyBStard's Avatar
 
Join Date: Feb 2008
Location: York, UK
Quote:
Originally Posted by tiefbassuebertr View Post
this is typical, because there are low currents and high impedances. By MM RIAA head amps with tubes the same effects are to observe.
You can reduce the microfonic effects by the use of a very heavy board for putting up your RIAA device (at best sandwich structure, inside a mixture of lead shut and sand for birdcages).

How much is the noise? More or few as that one from the neutral groove from your record disc?

Noise like a badly tuned radio indicates oscillation (an introduce of an gate stopper resistor of 1-5 K-Ohm near at the Gate of the input jFET can help)

If by tap on the metal case I hear a sound like a metal case, it is necessary, to coated your plates with bitumix or similare suited damper stuff (the ventilation and thermal management shall not be influenced !!)

For the ultimate sonic quality microphonic must be greatly reduced.

Therefore, a certain sound quality level isn't break with tube technology. And therefore tube technology by RIAA head amps is only recommend by low and medium cost devices and not for top class products.
Where the drain resistors specify 2k4 I have used 2k7 and the FETs are matched at about 6.4-6.5 mA, could reducing these drain resistors help?
  Reply With Quote
Old 9th February 2010, 05:47 PM   #4
diyAudio Moderator Emeritus
 
Iain McNeill's Avatar
 
Join Date: Oct 2007
Location: Santa Cruz, California
I very much doubt it. As tief says, with such high impedance the leakage capacitance from the oc input to the case is sufficient to create a signal. As the case vibrates, the small change in distance changes the capacitance value, the charge is constant so there must be a voltage change. Gain it up 60dB and hey presto - you just made a microphone.

If you calculate the effective thermal noise generated by a resistor equivalent to your cartridge, does it jive with what you're seeing for noise?
  Reply With Quote
Old 9th February 2010, 06:00 PM   #5
diyAudio Member
 
JammyBStard's Avatar
 
Join Date: Feb 2008
Location: York, UK
Funny thing, now I've had it running for a couple of hours the noise has stopped, I turned it off for half an hour, turned it back on, the noise came back and then faded out after a few seconds and seems to be running silent now; Well I'm confused, I'll keep an eye on it for a while.
  Reply With Quote
Old 9th February 2010, 06:02 PM   #6
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
I forget to mention a second thing: the circuit from post #1 is castrated "Pearl" RIAA head amp - basicly the same topology, namely a passive RIAA network between two line stages with voltage gain.
http://www.passdiy.com/pdf/pearlphono.pdf
For me in general the royal way to get ultimate audible sonic quality.
The reason for very fine results by the "PEARL" phono stage is not the use of cascode topology, but the paralleling connected jFET devices, especially for the input. So you get more idle current and lower noise.
Because of the higher current flow at whole the RIAA network can have lower impedance (i. e. lower resistor values and higher capacities)
The first step for you must be to check out, if there are oscillation or not by the follow three modes:
1) input shortet
2) input open (only the presently input resistance)
3) cartridge coil as input load (normal mode)
In none of these modes oscillation may occur, otherwise there are errors regarded the leading (Cable Management) and layout - unfortunately mostly hard to find. If I see the kind of build, my assumption is quite likely.
If you use for drain resistor 2k4 instead 2k7, you will not have appreciable effects.
But I missed the neg. voltage for exactly gate bias adjusting the middle point of voltage

Last edited by tiefbassuebertr; 9th February 2010 at 06:23 PM.
  Reply With Quote
Old 9th February 2010, 06:44 PM   #7
diyAudio Member
 
JammyBStard's Avatar
 
Join Date: Feb 2008
Location: York, UK
Quote:
Originally Posted by tiefbassuebertr View Post
But I missed the neg. voltage for exactly gate bias adjusting the middle point of voltage
Cheers Tief, but what do you mean here?
  Reply With Quote
Old 9th February 2010, 07:15 PM   #8
diyAudio Member
 
tiefbassuebertr's Avatar
 
Join Date: Jul 2005
Location: D-55629 Schwarzerden
Quote:
Originally Posted by JammyBStard View Post
Cheers Tief, but what do you mean here?
I mean the adjust for symmetric clipping respectively for adjust the lowest distortion if the voltage swing is in the aera below 1Vss
The great advantage now is the possibility to use many different jFETs independend of their individual gate-source threshold voltage values.
By use of paralleling connected jFET devices each jFET get it's own adjust. Now you must only select of identical transfer curvatures - individual threshold voltage values are now no matter.
By the way - a useful update for new "PEARL" projects.
Attached Files
File Type: pdf C ...-UNTITLED.CKT.pdf (8.9 KB, 21 views)

Last edited by tiefbassuebertr; 9th February 2010 at 07:25 PM.
  Reply With Quote
Old 10th February 2010, 03:33 PM   #9
Sheldon is offline Sheldon  United States
diyAudio Member
 
Join Date: Dec 2003
Location: San Diego
Since you already built one version of this preamp, and it didn't suffer from the noise effects that you describe here, then I would look at the differences first.

You have two issues; noise and microphonics. They may have a common cause, maybe not.

I assume the noise and microphonics are the same for both channels?

Some of the noise could be in your power supply. Looks like you've bypassed them, the regulators too? Batteries can produce some noise, and this is a high gain device.

Some caps can be microphonic, if they are not wound tightly or leads are not firmly attached. Those caps don't look suspect, and I would put this issue low on the list.

Poor solder joint can lead to noise and microphonics. This could be an issue if the noise is localized to one channel especially.

Different layout could lead to oscillation or more microphonics than the first version.

Are any connections longer than your first version, especially to the FET gates or drains. As Tief. mentioned, try a stopper resistor.

Is the metal case earthed?

The microphonics could be just a mechanical issue. Tap gently with a non conductive probe and see where the effect is loudest. Look at your component and joints there. You may be able to isolate that issue.

Sheldon

Last edited by Sheldon; 10th February 2010 at 03:51 PM. Reason: more careful reading of initial post
  Reply With Quote
Old 10th February 2010, 03:42 PM   #10
Sheldon is offline Sheldon  United States
diyAudio Member
 
Join Date: Dec 2003
Location: San Diego
Quote:
Originally Posted by tiefbassuebertr View Post
If you use for drain resistor 2k4 instead 2k7, you will not have appreciable effects.
No effect in the second position, but a small effect on the RIAA curve for the input drain resistor. In either case, no influence on noise or microphonics.

Sheldon

BTW carbon comp resistors will be noisier and less stable than wirewounds. Wirewounds are the least noisy, carbon comp. the most.

Last edited by Sheldon; 10th February 2010 at 03:44 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Mobile Phone Interference gingertube Tubes / Valves 14 6th October 2009 10:20 PM
tube pre amp for phone stage hunsoop Tubes / Valves 15 1st May 2009 01:31 AM
Power Supply For Pre/ Head Phone & Phantom... (PCB) Ayne Power Supplies 0 28th April 2007 04:10 PM
Toroidal transformer bolt question & mobile phone interference markiemrboo Chip Amps 19 5th December 2006 11:15 PM
Pre-pre; phono; pre line ;device discrete (bjt/fet) o op. amplifier GIPIONE Solid State 8 18th December 2005 09:09 PM


New To Site? Need Help?

All times are GMT. The time now is 02:30 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2