Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Analogue Source Turntables, Tonearms, Cartridges, Phono Stages, Tuners, Tape Recorders, etc.

Uher CR240 hiss in output stage/general noise reduction
Uher CR240 hiss in output stage/general noise reduction
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 8th November 2019, 02:47 PM   #1
Kaidanovsky is offline Kaidanovsky  United Kingdom
diyAudio Member
 
Kaidanovsky's Avatar
 
Join Date: May 2008
Location: Cornwall
Default Uher CR240 hiss in output stage/general noise reduction

Ahoy,

I'm in the final stages of restoring this having made a new switch and repaired a main board crack and the aftermath of a small fire among many other things.

With the unit on, nothing in circuit except the output ICs, and volume on minimum there is significant hiss in the headphones.

As a transistor neophyte, feeling my way, I wondered if anyone had any suggestions as to where the constant output stage noise is coming from and what I could do to mitigate it in an existing circuit.

Attached is the diagram - the previous IC and all other boards are removed so there is no input. If I ground the input pins of each channel, there is a buzz somewhere between motorboating and mains hum.

I notice the datasheet of the op-amp contains more capacitance on the power line. I think this could be a culprit, as if the motor is engaged (with no connection between the heads and the output) there is hum added to the hiss. While the diagram says C37 is a 1F/35V, the tantalum capacitor in circuit was 0.1F (I replaced it with polyester).

There is 4.6VDC on the output, reduced to 3mV after the coupling capacitors.
Attached Images
File Type: png CR240_Output_stage.PNG (234.9 KB, 43 views)
File Type: png TBA820.PNG (44.2 KB, 45 views)
__________________
Providing sparkling relief to the palate and keen gratification to the whole wilted system.
  Reply With Quote
Old 9th November 2019, 11:54 PM   #2
Mark Tillotson is offline Mark Tillotson
diyAudio Member
 
Join Date: May 2018
Location: Cambridge UK
I'd use ceramic for that 0.1F decoupler so it works at RF. I'd add lots more electrolytic decoupling, the TDA820 has very poor PSRR. Its also pretty noisy, but for a cassette deck you probably don't care about that.
  Reply With Quote
Old 10th November 2019, 01:01 PM   #3
Kaidanovsky is offline Kaidanovsky  United Kingdom
diyAudio Member
 
Kaidanovsky's Avatar
 
Join Date: May 2008
Location: Cornwall
That's interesting, Mark. Perhaps there wasn't a better chip available in 1979!

Reading up on today's offerings, I see that 100dB PSRR is the norm, whereas the TBA820 has 40dB! Perhaps I could fit a more modern 14-pin example in its place?

The rail comes from the PSU with no decoupling in between except that 0.1F I took out (having read about the problems of tantalum decouplers), so it can't be helping. I'm limited by space in the chassis as the loudspeaker sits just above the TBA820s when the thing's screwed together, but I can certainly fit the datasheet's suggested 1F and the ceramic.

Is the hiss in this case then simply the various noises of the component itself? I'll see if it changes when the RF is reduced.

Through the loudspeaker it's not noticeable for monitoring, and I've not tried the output to an external amplifier yet. It's just very obvious in the headphones, which for a high-end 'professional' machine of the day isn't so hot for recording.

I could try reducing the gain as well - they're plenty loud enough at very low volumes.
__________________
Providing sparkling relief to the palate and keen gratification to the whole wilted system.
  Reply With Quote
Old 12th November 2019, 04:04 PM   #4
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
I think your problem is twofold:
1. Mediocre PSRR as suggested capacitor C6 (Fig. 2) is not installed.
2. Almost no attenuation for the headphone output, combined with an IC sporting 3 V of input voltage noise at a gain of 36 dB (output noise ~185 V - an average-performing headphone amplifier will exhibit 20-30 V, a good one <2 V).

ICs like the TBA820 were a common sight e.g. in clock radios of the time. In this device it may have been chosen for its low power consumption.

The easiest way of taming the headphone output (short of using an external attenuator) would be changing the values of R74/75 and adding two more from each output to ground (a good low-impedance ground return), so that a practical attenuation of maybe 16-20 dB would be achieved. What kind of impedance and sensitivity are your headphones? For sensitive 32 ohm cans I might use 120 ohms followed by 12 ohms, for typical 250-300 ohm Beyerheisers more like 330-390 ohms and 47 ohms, and for only moderately sensitive old 600 ohm jobs perhaps 330 ohms and 100 ohms.

Adding a little dedicated headphone amplifier board suitable for single supply operation may be another option, though this may be hard to fit into such a compact device.
  Reply With Quote
Old 15th November 2019, 01:56 AM   #5
Kaidanovsky is offline Kaidanovsky  United Kingdom
diyAudio Member
 
Kaidanovsky's Avatar
 
Join Date: May 2008
Location: Cornwall
Thank you for the thoughts.
Quote:
Originally Posted by sgrossklass View Post
I think your problem is twofold:
1. Mediocre PSRR as suggested capacitor C6 (Fig. 2) is not installed.
I thought this was strange too, comparing the diagrams. I can't think why the designers didn't want to get the best monitoring output they could, which would surely include reducing ripple. Perhaps mains power was considered less common than battery. I will mainly be using it on the mains.

Quote:
Originally Posted by sgrossklass View Post
2. Almost no attenuation for the headphone output, combined with an IC sporting 3 V of input voltage noise at a gain of 36 dB (output noise ~185 V - an average-performing headphone amplifier will exhibit 20-30 V, a good one <2 V).

ICs like the TBA820 were a common sight e.g. in clock radios of the time. In this device it may have been chosen for its low power consumption.
That could be the case - as it is, I understand from the specifications that it isn't easy on batteries! I haven't found a modern drop-in replacement from my usual suppliers, so I think I'll work with this and the noise-reduction ideas in the interests of DIY, originality and cost.

Quote:
Originally Posted by sgrossklass View Post
The easiest way of taming the headphone output (short of using an external attenuator) would be changing the values of R74/75 and adding two more from each output to ground (a good low-impedance ground return), so that a practical attenuation of maybe 16-20 dB would be achieved. What kind of impedance and sensitivity are your headphones? For sensitive 32 ohm cans I might use 120 ohms followed by 12 ohms, for typical 250-300 ohm Beyerheisers more like 330-390 ohms and 47 ohms, and for only moderately sensitive old 600 ohm jobs perhaps 330 ohms and 100 ohms.

Adding a little dedicated headphone amplifier board suitable for single supply operation may be another option, though this may be hard to fit into such a compact device.
Interesting - I have both ends of the spectrum there, but more likely to use the 32 Ohm for listening at home. Is the intention to create a voltage divider for permanent attenuation? Am I right in deducing that at present the resistors are just there for approximate impedance matching for 32 Ohm headphones?
__________________
Providing sparkling relief to the palate and keen gratification to the whole wilted system.
  Reply With Quote
Old 16th November 2019, 09:55 PM   #6
sgrossklass is offline sgrossklass  Germany
diyAudio Member
 
Join Date: Nov 2004
Location: Germany
Quote:
Originally Posted by Kaidanovsky View Post
Interesting - I have both ends of the spectrum there, but more likely to use the 32 Ohm for listening at home. Is the intention to create a voltage divider for permanent attenuation?
Indeed. You want to find a good compromise between output impedance and noise / remaining maximum output level. To complicate matters, sensitivity to output impedance varies quite a bit between headphones even at same nominal impedance - you can do the math if you have an impedance response for the model in question. (A handful of models like a few old Beyers actually sound best at a non-zero output impedance.) Likewise, sensitivity also varies a fair bit.
Quote:
Originally Posted by Kaidanovsky View Post
Am I right in deducing that at present the resistors are just there for approximate impedance matching for 32 Ohm headphones?
Your guess is as good as mine. Headphones were at least as much of a mess in 1977 as they are now. I would like to think that the developers' cans of choice may have been the trusty 2 kOhm Sennheiser HD414 or HD424 (the 600 ohm versions only came out in the very late '70s I think) rather than their 8-25 ohm Japanese counterparts, but who knows. As-is, the output is inadequate for almost anything I can think of.
  Reply With Quote

Reply


Uher CR240 hiss in output stage/general noise reductionHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Harman/Kardon HK6500 - white noise (hiss) in output after recap ThatDavid Solid State 0 6th February 2019 09:23 PM
Amp with cassette hiss reduction using common IC(tba810/tda2003) indr Chip Amps 1 26th June 2016 07:59 PM
common value of output stage noise in uV? rhythmsandy Solid State 4 14th March 2014 10:00 PM
Noise reduction on output of SMPS cod3gen Power Supplies 3 24th October 2013 03:09 PM


New To Site? Need Help?

All times are GMT. The time now is 03:46 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki