Stereo Demodulator issue. Baffled!

[LucasAdamson]

Originally Posted by 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.

Dave, mine is the later DC coupled, capacitor-less DCB1. I will try some DC blocking series caps at the output of the radio next.

[LucasAdamson]

Originally Posted by 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

Thanks Peter, I'll try some caps at the output. I'll also consider lowering the value of R42.

[G8HQP Dave]

Ok, I didn't realise that there are two designs with the same name. If your power amp is also DC-coupled then you may have trouble. In most setups every item has coupling caps in case someone omits them, so half of them are unnecessary. Your setup may be the exception which proves the rule - none of them have caps because each assumes that all the others will do DC-blocking.

[PJC58-Hythe]

You said the Pass B1 buffer is "DC Coupled" is this correct, as I assume you mean the signal path? Different from the circuit Dave has posted a link to?

I agree with Dave that a notch filter would be good, or a low pass filter with a steep roll off above 20Khz.

Have you tried the output from the decoder into any other amplifier to see if you get the same effects?

[LucasAdamson]

Originally Posted by G8HQP Dave

Ok, I didn't realise that there are two designs with the same name. If your power amp is also DC-coupled then you may have trouble. In most setups every item has coupling caps in case someone omits them, so half of them are unnecessary. Your setup may be the exception which proves the rule - none of them have caps because each assumes that all the others will do DC-blocking.

Do you know what, you right well be right. My pre and power amps are both DC coupled. My DAC isn't, and has 2uF caps at it's output. My phono stage also isn't and has output caps. Of course, when making the amps I checked for DC offset, but not on this radio. I'll look into it after the kids have gone to bed ( I don't like measuring a live valve radio before they're safely in bed.) So, could it be that some DC is inaudible, but then suddenly as I turn up the volume it reaches saturation point?

[LucasAdamson]

Plugged it into a Pioneer A300X integrated amp, it plays fine, but when I crank the volume up, I get the hum, and then the speaker protection circuits kick in, so it seemed pretty certain to be DC we are dealing with here.

Then, I measured it. In MONO mode, we are talking about 0.14mV(ish) per channel. Perfectly acceptable of course.

In STEREO, we are dealing with 4.3vDC on the left phono and 4.11vDC on the right. OK, so that's it then. Many thanks to you all.

Anybody care to suggest an optimum cap value? The theory on DC blocking caps is staggeringly complicated.

[DangerMan]

That sounds potentially very nasty!
If your entire amplifier chain is DC coupled and you feed in DC to the input, the speakers will get a good dose of DC applied to them. The audio will sit on top of the offset, so there is still an output, but the bass drivers will be displaced from their normal position and, much worse, the current will quickly heat up the voice coils which won't do them any good at all !

Pete

[PJC58-Hythe]

If your DAC has 2uF caps on the output and that works fine, use the same value on the stereo decoder outputs. I just hope you haven't done too much damage to those voice coils!

Peter

[LucasAdamson]

I don't think I've burned out any of the coils, judging by the sound. They should be OK.

I will try 2uF, as that often seems to be value chosen for this purpose. I have no idea about the deciding factors in this though - ie how much capacitance for how much DC to remove.

[G8HQP Dave]

No connection whatsoever between capacitance value and how much DC to remove - that just affects the required capacitor max voltage.

The cap value is set by the desired low frequency rolloff as the cap and the resistance of whatever follows form a CR high pass filter. Google will help you.

[LucasAdamson]

OK, so I had some very old (1970s) 2uF polyester bipolar speaker crossover caps left over from a speaker project. I bunged them in, and the DC is gone, but....

The amp now sounds like any other horrible crappy cheap transistor tuner. Gone is the lovely lush sound which was there when I had 4.5vDC hacking away at my speakers. Before these caps, it was just as the MONO channel, lovely and lush, rich and smooth, now it's R-O-U-G-H. Now, I know that these caps are a bit crap, even though they are polyester - I think they're the type where they spray metal particles onto polyester and then roll it up, not real rolled foil in oil types, but I cannot live with them. Anyway....

I had bought two Silver Mica 850pF caps for using as de-emphasis replacements for the stereo channel - would they work well for DC blocking as well? Will they attenuate low frequencies too much? They are quite tiny values but I am told excellent quality for signal.

The first resistors in the pre-amp are a 220K shunt and a 220R series, if that helps determine the effect of the CR.

I still don't quite get why the DC wasn't ever present, but only came on suddenly when the volume was in the 9pm position.

Many thanks radio dudes,
Lucas

[Synchrodyne]

Originally Posted by LucasAdamson

I still don't quite get why the DC wasn't ever present, but only came on suddenly when the volume was in the 9pm position.

I think that the DC was always present at the power amplifier input, but attenuated by the volume control so that at low volume settings it didn't do any harm, or at least audible harm.

As a sidebar, I see that the B1 buffer is described as not having negative feedback:

"Given the simple task, it's pretty easy to construct simple buffers with very low distortion and noise and very wide bandwidth, all without negative feedback."

Ummm....doesn't a follower (whether cathode follower, emitter follower, source follower, or voltage follower) have more-or-less 100% negative feedback, or I am missing something here?

Cheers,

[Synchrodyne]

Originally Posted by 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.

To add to what G8HQP Dave has said, I have a vague recollection that FET source follower inputs can be a bit susceptible to extraneous out-of-band signals such as RFI, which end up being rectified. I think this was reported for the very early Ferrograph Series 7 tape decks, which had FET line input buffers for high input impedance, to allow both direct connection of ceramic cartridges and to match valve amplifiers whose tape outputs typically needed to look into 500k or higher. I am not sure if I can find the original reference, though, and I might not be remembering the problem correctly. And buffer stage behaviour does not seem to be the primary issue here.

Cheers,

[LucasAdamson]

Originally Posted by Synchrodyne

I think that the DC was always present at the power amplifier input, but attenuated by the volume control so that at low volume settings it didn't do any harm, or at least audible harm.

As a sidebar, I see that the B1 buffer is described as not having negative feedback:

"Given the simple task, it's pretty easy to construct simple buffers with very low distortion and noise and very wide bandwidth, all without negative feedback."

Ummm....doesn't a follower (whether cathode follower, emitter follower, source follower, or voltage follower) have more-or-less 100% negative feedback, or I am missing something here?

Cheers,

You're probably missing something. He's a proper genius, Nelson Pass. Anyway, this is the DC coupled version - it's a fair bit different.

Yes, DC was ever present, but it still amazes me that it sounded lovely and then suddenly Bzzzzzzzzzzzz.

[Billy T]

Originally Posted by LucasAdamson

Anyway...I had bought two Silver Mica 850pF caps for using as de-emphasis replacements for the stereo channel - would they work well for DC blocking as well? Will they attenuate low frequencies too much? They are quite tiny values but I am told excellent quality for signal.

850pF is not appropriate for use as an audio coupling cap, you will have limited HF and no LF to speak of. They are nice and stable for RF, and useful perhaps for HF roll-off in HiFi treble control ciruits and FM de-emphasis as you know from your original purchase purpose.

The best coupling cap value will be determined by the impedance of your circuits, but I don't think there is enough information available yet.

Cheers

Billy

[murphyv310]

I still have a sneaky feeling it is to do with your supply. Perhaps I have missed a posting but have you isolated the supply completely from the tuner and fed the decoder from an external supply, even batteries would be fine for a quick test!

Why not feed your new decoder from the existing decoder feed and if necessary build a little stabiliser with an LM317.

[TuningIndicator]

I agree with Trevor. (And Al)

See posts (Al) no3, (Trevor) no4, and (my post) no8.
The OP seems to be ignoring the power supply, and considering everything but.

A power supply is the root of everything; power supply problems can give rise to the weirdest of faults.

[LucasAdamson]

It was DC in the signal, now blocked with 2x 6,3uF polypropylene caps. The radio no longer distorts or gives hum beyond 9pm position, as the DC is gone, but it is still VERY loud. Quite why, I still haven't yet determined. It seems that the circuit diagram for UA758 is either incomplete or makes the assumption that DC blocking will be present in the amp signal input (which would normally have been the case to be fair). The loudness is another matter. The power supply, you say wisely. I will try an external battery source and report back.

[LucasAdamson]

Originally Posted by 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?

Thanks for that advice. My voltage doubler was recommended by someone who had achieved success with the same thing. All of the advice given to me on this forum was to ignore the 24v supply and use this 6.3vAC winding, as it carried more (required) amperage. My voltage doubler is two 6800uF caps (overly large, I know) and two diodes, in the normal configuration for such a rectifier. It results in 15.8vDC to the multiplex decoder. Are you suggesting that two further RC stages would help me here in some way?

Many thanks
Lucas

[G0PKH - Pete]

Hi

The circuit diagram of the UA758 certainly does look incomplete.
It would seem that it is simply an application circuit design, rather than a full constructional design. There is no thought given to interfacing to an amplifier.

If I were trying this. I would start by coupling the audio to an amp via a pair of 10uF electrolytics (one per channel of course and +ve to the UA758 outputs where the 4.5ish Volts are) this will remove the DC content.

Then if the output is still too loud try a potential divider from the -ve end of the caps to deck, and take the output from the center of the divider.
In fact a 100K preset pot as the divider (take the output from the wiper), that way you can balance the levels to match that of the MONO output.

Pete