Stereo Demodulator issue. Baffled!

[LucasAdamson]

OK, so I have made and installed my new UA758 based Stereo Decoder in my Leak Troughline Stereo, and it works well up to a point. Kind of a big High 5 moment for me! I made it to fit in place of the old solid state de-modulator, in the small box inside the radio. It now runs from the 6.3vAC heater winding, by using a voltage doubler to get 15.8vDC

I get Stereo radio right across the board. LED indicates Stereo tuning.

Reception signal is not perfect, but pretty ok considering I'm using a tv aerial.

The sound quality is very good indeed. Especially on Radio 3 and 4.

The problem now:

Mono signal is very quiet. I have to turn my amp right up high to get a decent listening level. I just replaced C50 mono de-emphasis cap (which was 4000pF, suggesting it is a US model) with 1000pF, as recommended on this forum to remove attenuation from high frequencies.

By contrast, Stereo output signal is very high, and this is the weird bit:

If I turn my amplifier up beyond the 1/4 way mark, at one point it suddenly turns from nice loud music to a large loud constant hum, which remains at a constant level as I turn the volume knob further.

Can anybody explain what is likely going on here? The amp is in perfect condition (I made it myself) and functions perfectly for all other sources, including the mono mode of this radio.

Many thanks
Lucas

[Sideband]

Maybe if we could see some circuits.....? particularly how you've connected the decoder to the tuner and the volume control.




SB

[Skywave]

Sounds like unwanted, unintentional earth loops to me. May be related to the fact that you are using the 6.3-v.a.c. lines to source your power rail for the UA758.

Al.

[murphyv310]

I think Al has hit the nail on the head. Try feeding the decoder from a separate DC power source and see if the fault goes away.

[LucasAdamson]

Thanks guys.

Can you explain to me how it happens that it plays fine (well, in fact) until I turn my amp up above 1/4 way....that's the bit that totally baffles me. They are connected only at the mains, by sharing an extension socket and by the phono connectors.

Also, the use of the heater filament winding is a recognised method of powering precisely such a decoder in this model, and has been reported to work in the past. How would my amp volume position even affect this?

Many thanks
Lucas

[LucasAdamson]

See schematics:

Leak Troughline Stereo:
http://www.44bx.com/leak/Leak/Circui...ghLine2_d2.gif

UA 758 Multiplex Decoder:
http://i25.photobucket.com/albums/c5...evol/ua758.jpg

[LucasAdamson]

Originally Posted by Skywave

Sounds like unwanted, unintentional earth loops to me. May be related to the fact that you are using the 6.3-v.a.c. lines to source your power rail for the UA758.

Al.

Thanks Al,

Could earth isolation with a CL60 thermistor help out here?

Many thanks
Lucas

[TuningIndicator]

I think this is power supply related, as pointed out by Trevor and Al.

You've taken the 6.3VAC valve heater supply and stuffed a voltage doubler on it, and then powered your decoder with the resulting DC voltage?

If you look at the original power supply in the Leak drawing, a 24VDC rail is provided for the original decoder, and passes first via the main CRC filter, and then two further RC stages, before reaching the decoder.

Have you done anything similar?

[MrElectronicman]

You might try putting in a couple of capacitors in the stereo outputs from the decoder, since there maybe a residual DC voltage out of the decoder that may affect the input to the amp.

The fact that the mono signal is low may be due to the switch, since on my tuner the mono output is higher than the stereo and I had to put a trimmer pot in order to match the levels.

[Skywave]

Unintentional earth loops - especially those that are embedded within an item, not external mains connections to the item - can be responsible for a whole raft of strange (and usually unwanted) behaviours. For example, in your case, there may well be a circulating earth current from the PSU that also includes an earth current in the same path from the volume pot. A certain amount of intermodulation is almost certain to be taking place between these two currents, (solder joints, nut & screw connections, etc.) and when the volume control hits a certain setting, the magnitude and relative phase of these currents - and the extent of any intermodulation - could easily rapidly change and cause a distinct and sudden non-linearity in the earth currents distribution - so you suddenly hear lots of hum.

The only way to be really sure is to open up the items of equipment that are so inter-connected, examine closely what and where the earthing arrangements are, and redo these earthing arrangements (if necessary) in line with standard good practice for such earthing. Usually, this boils down to all earths, power and signal, being returned to one point only - and this is usually one side of the PSU filter cap., usually -ve, for a +ve PSU rail. Oh, yes, and it's a good idea to run the incoming mains safety earth line to this same point as well.

Finally, tracing this type of fault is one where using an oscilloscope is no help whatsoever! Simply because not only will the location of the 'scope probe earth clip change the test situation, (you've introduced another earth connection), but that the mere connection of that earth clip could cause the hum (or whatever) to disappear! Or get worse!

Al.

[LucasAdamson]

That's really helpful Al. Of course it has to be an earthing issue when I think about it.

From acquiring the Leak, I have done the following:

1. Replaced the mains cable with a twin and earth, for safety reasons. The whole amp was un-earthed, and the entire chassis is negative return earthed, like a car, and what with the point to point circuitry and loving my kids and all, I thought an earth connection might be in order. I soldered it to the chassis - a random place, as there are literally dozens of earth points on the chassis - all random, as part of this design.

2. Replaced 2 or 3 very old electrolytes - I measured them first, and replaced them with near exact replacements.

3. Removed and replaced the Multiplex Decoder with UA758 chip. Powered it from 6.3v winding with voltage doubling rectifier with 2x 6800uF 35v caps (large, I know, but I had them spare) and 2x medium sized diodes.

4. Replaced 4000uF de-emphasis cap in Mono circuit with 1000uF Silver Mica, as per recommendation from expert on this forum.

I now intend to isolate the voltage doubler from chassis earth by connecting diode/cap to earth via a CL60 thermistor, which has helped me out with earth loops in the past (a top tip from amp design guru Nelson Pass). I'll let you know how I get on...

thanks again!

[LucasAdamson]

Originally Posted by MrElectronicman

You might try putting in a couple of capacitors in the stereo outputs from the decoder, since there maybe a residual DC voltage out of the decoder that may affect the input to the amp.

The fact that the mono signal is low may be due to the switch, since on my tuner the mono output is higher than the stereo and I had to put a trimmer pot in order to match the levels.

Thanks, I will check for DC on the output, and if I find it, I'll be sure to cap it off, so to speak.

[Synchrodyne]

Some thoughts:

The new decoder could well have different gain to the original, which would account for some difference in output levels between mono and stereo.

What is the input configuration of the associated amplifier? Is the radio input buffered? Or does it go direct to the volume control?

The tuner mono output probably wants to look into a highish impedance – say 50 k or so (100 k each channel), maybe more, bearing in mind that left and right amplifier inputs are fed in parallel from the tuner mono output. If the impedance is too low, the signal level will be pulled down and the de-emphasis curve could be altered. The mono output is taken from a 150k over 68k divider, and the amplifier input impedance is in parallel with 68k. So an amplifier input impedance that is low, say down around 20k or less, will have an effect.

If I recall correctly the µA758 decoder IC has emitter follower outputs (unlike the otherwise similar MC1310 which does not) so should be less fussy about what impedance it looks into. But even so, reasonably high is probably better.

If both the tuner and amplifier are earthed, then there is a potential hum loop. Whether or not this is a factor could be checked by inserting a two-channel audio isolating transformer in the interconnection between tuner and amplifier. But as one would expect any ill effects to be the same on both mono and stereo, this might not be the first place to look for problems.

If one signal lead (left or right) is disconnected, is the hum effect still present at ¼ volume and up, or does the trigger point change?

Possibly, if the tuner is looking into a very low amplifier input impedance that gets noticeably lower as the volume control is advanced, a point is reached where the increased loading on the µA758 output triggers a latent problem related to power supply impedance, ripple rejection capability, etc. Very low amplifier impedance would also explain the low mono level issue, and if input impedance decreases with increased volume setting, a point could be reached where turning up the latter could have but minor effect. Far-fetched maybe, but perhaps worth eliminating from the list of possibles? Or on the outside chance that it is the problem, buffering each of the mono, left and right outputs, or buffering the amplifier inputs might help.

Cheers,

[Valvepower]

Hi,

This is a total long shot but worth a punt …. There appears to be no extra filtering on the output of the decoder? There is a de-emphasis filer but nothing removing the (19 or 38K ..?) switching noise, if there is any that is? This means the decoder could be feeding residual HF switching noise from the decoder into the amplifier, which the amplifier can deal with at low volume settings, but as you advance the volume it ‘tilts’ the balance and thus causing the amplifier to go unstable making a humming noise?

This switching noise used to cause problems ‘beating’ against the Erase/Bias oscillator on tape recorders.

If you look at the Quad FM3 for example there is an 18dB/Oct filter on the output.

Otherwise it does sound like a ground, earth or power supply ripple/regulation problem.

Terry.

[stuarth]

I agree with Valvepower, this appears to be an instability issue.

The fact that there is a "trigger point" for the change in behaviour certainly points that way.
It could be feedback from the amplifier output back into the tuner - do the speaker wires run close to the tuner or aerial wires?

What is the amplifier design? Some solid-state amplifiers use 100MHz + transistors in a design which only considers AF performance. You need to consider the amplifier response to RF. In the setup described here, there is a HF rolloff in the mono position (the 150k/68k resistors and 1000pF capacitor will stop RF passing from the tuner to the amp) which is not present in the stereo position. You could try adding a filter on the stereo output, eg the 47k/3000pF shown in the Leak decoder. This could also fix the gain difference between mono and stereo.

If this cures the hum/stability problem, you might look into the RF response of your amplifier.

Stuart

[LucasAdamson]

Originally Posted by Synchrodyne

Some thoughts:

The new decoder could well have different gain to the original, which would account for some difference in output levels between mono and stereo.

What is the input configuration of the associated amplifier? Is the radio input buffered? Or does it go direct to the volume control?

The tuner mono output probably wants to look into a highish impedance – say 50 k or so (100 k each channel), maybe more, bearing in mind that left and right amplifier inputs are fed in parallel from the tuner mono output. If the impedance is too low, the signal level will be pulled down and the de-emphasis curve could be altered. The mono output is taken from a 150k over 68k divider, and the amplifier input impedance is in parallel with 68k. So an amplifier input impedance that is low, say down around 20k or less, will have an effect.

If I recall correctly the µA758 decoder IC has emitter follower outputs (unlike the otherwise similar MC1310 which does not) so should be less fussy about what impedance it looks into. But even so, reasonably high is probably better.

If both the tuner and amplifier are earthed, then there is a potential hum loop. Whether or not this is a factor could be checked by inserting a two-channel audio isolating transformer in the interconnection between tuner and amplifier. But as one would expect any ill effects to be the same on both mono and stereo, this might not be the first place to look for problems.

If one signal lead (left or right) is disconnected, is the hum effect still present at ¼ volume and up, or does the trigger point change?

Possibly, if the tuner is looking into a very low amplifier input impedance that gets noticeably lower as the volume control is advanced, a point is reached where the increased loading on the µA758 output triggers a latent problem related to power supply impedance, ripple rejection capability, etc. Very low amplifier impedance would also explain the low mono level issue, and if input impedance decreases with increased volume setting, a point could be reached where turning up the latter could have but minor effect. Far-fetched maybe, but perhaps worth eliminating from the list of possibles? Or on the outside chance that it is the problem, buffering each of the mono, left and right outputs, or buffering the amplifier inputs might help.

Cheers,

My pre-amp is a selector for 4 channels (using relay switches), a DC coupled Pass B1 buffer (signal is buffered only), and the volume is by 2 series and 2 shunt resistors that are light sensitive (LDRs) optically coupled to 4 leds, powered by a regulated 5v supply, controlled by the volume control, which actually only controls voltage to the leds. The signal is very short indeed, and all wired in silver (I know - it's a bit overkill). So, the signal goes through a relay (no effect) a B1 buffer (input impedance is theoretically 1M, output is very low - that's the whole point of it.)

[LucasAdamson]

Originally Posted by stuarth

I agree with Valvepower, this appears to be an instability issue.

The fact that there is a "trigger point" for the change in behaviour certainly points that way.
It could be feedback from the amplifier output back into the tuner - do the speaker wires run close to the tuner or aerial wires?

What is the amplifier design? Some solid-state amplifiers use 100MHz + transistors in a design which only considers AF performance. You need to consider the amplifier response to RF. In the setup described here, there is a HF rolloff in the mono position (the 150k/68k resistors and 1000pF capacitor will stop RF passing from the tuner to the amp) which is not present in the stereo position. You could try adding a filter on the stereo output, eg the 47k/3000pF shown in the Leak decoder. This could also fix the gain difference between mono and stereo.

If this cures the hum/stability problem, you might look into the RF response of your amplifier.

Stuart

Hi Stuart,

Thank you for your post, which makes a lot of sense.

I did wonder about the de-emphasis filter from the stereo circuit, but looking at the schematic, there is one incorporated into the UA758 circuit. If you look at the circuit I used, it there are 2 identical de-emphasis filters coming off the chip and re-entering. I made this circuit as per the schematic seen here. They may be the wrong values for this amp though - they are 3.9K and 0.022uF

[MrElectronicman]

Can't see that the de-emphasis would cause this kind of problem. If the values were wrong the sound would either be muffled or high pitched.

It also occurred to me that the hum may be an earth problem. If you have both the amp and the tuner connected to the mains via a 3 core cable, and then you connect both the tuner and the amp together via the signal cables then you have a earth loop.

Also the 6.3 VAC is grounded at one side, but if you also grounded the decoder at a different point , you would get another earth loop.

On a really long shot, if the wires from the 6.3 VAC were reversed, you could end up with the 12 VDC being earthed at the positive rather than the negative, I said this is a long shot, but could cause all kinds of strange behavior.

[PJC58-Hythe]

Just a couple of things... What is the input impedance of your amp? In mono there is efectively 150K in series with the input to your amp (R42) so unless it's very high there is going to be significant signal loss across this resistor.

The output from the UA758 is low impedance and will have no problem driving a much lower impedance load. More critically the output from the UA758 is DC coupled - is the input to the amp AC or DC coupled? If the latter as you wind up the volume I would suspect that there is a larger offset that is causing the output on the power amp to swing heavily +ve or -ve and this maybe your problem.

If you look at the datasheet for the UA758 you will see it can have upto 150mV (worst case) of offset at the output terminals. Try inserting a 10uF cap in series with each of the outputs on the UA758 and see if that fixes the problem.

Peter

[G8HQP Dave]

The Pass B1 buffer is simply a source follower (see here for the circuit). It has coupling caps and stopper resistors at both sides, so it should be reasonably free of interaction with adjacent equipment. I guess if you were very unlucky you could have instability, so try increasing the 1K stopper resistors.

Any remaining 38kHz subcarrier should not do any harm to the buffer, but it will pass it straight through to the amp so it could cause trouble there. De-emphasis will reduce 38kHz but not eliminate it - for that you need a notch filter. Put a scope on the decoder or buffer outputs, to see how much 38kHz there is.