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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 5th Apr 2019, 9:46 pm   #21
Robstan234
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Default Re: Leak Troughline Stereo low output

Hi, just a thought but have you tried the mic inputs on the tape recorders? or putting the stereo tuner output through a modern small mixer to get a higher output, the small Behringer mixers are reasonably quiet.
Rob
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Old 6th Apr 2019, 8:21 am   #22
Synchrodyne
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Default Re: Leak Troughline Stereo low output

Quote:
Originally Posted by Linnovice View Post
The only ones to receive a reasonable signal are the A77s. If I take the output of the Troughline direct to the Radio input of the A77 (via a din socket) I get a reasonable signal at the meter. I have been using one of my A77s as a pre amp and fed the signal, via a patch bay, out to the other decks. It works but seems like using a sledge hammer to crack a nut.
As I recall the Revox A77 Radio input (DIN) has a sensitivity of 2.5 mV and an impedance of 33k. So the result is as expected.

The 33k impedance, below the minimum recommended 47k, would result in some reduction of the output from the Troughline Stereo. It would also affect the de-emphasis curve. The net effect of moving the de-emphasis turnover point away from its correct point (3.18 kHz for 50 µs de-emphasis is to create a step in the frequency response curve. For example, if the turnover moved up by half an octave, then there would be a 3 dB step-up in the frequency response between 3.18 and 4.5 kHz. I imagine that this would be audible. One may wonder of some of the sound “qualities” attributed to the Troughline Stereo are due to “frequency bending” arising from mismatching than from any intrinsic properties. Small frequency response anomalies are probably the first things to look for when investigating apparent sound differences. Mismatching on the impedance front seems likely to occur when older sources are connected to modern control units.

Note that this comment applies only to the Troughline Stereo, which has unbuffered, high impedance stereo outputs. As far as I know, the Troughline, Troughline II and Troughline 3 all have buffered (cathode follower) outputs with the de-emphasis filters on the input side of the cathode followers, and therefore unaffected by output loading. The mono output of the Troughline Stereo is buffered, but on the output side it has a highish impedance attenuator to bring the level down to that of the stereo outputs, and the de-emphasis is done at this point. So it is likely to be affected by the load impedance that it sees.

Anyway, the Leak documentation states that the Troughline Stereo outputs should look into a load impedance in the range 47 to 100k. So quite simply that should be the objective, whether realized via a buffer amplifier or by a finding a piece of equipment that provides an input impedance in that range. A rule of thumb is that for a tuner, the amplifier (or other unit) input to which it is connected should have a sensitivity that matches or betters the output level at 30% modulation. In this case, that is 50 mV. So you need an input with a sensitivity of 50 mV (or a lower number) and an impedance in the range 47 to 100 k.

One solution might be to feed the Revox Radio input via a suitable resistive attenuator. Such could probably be arranged to keep the impedance seen by the Troughline Stereo above 47k. 68k, the geometric mean of the 47 to 100k range, is probably a suitable aim point. A 47k over 47k attenuator would do this. The bottom 47k is in parallel with the 33k input impedance of the Revox, giving a net 19k. So the Troughline will see (47+19) = 66k. The input level seen by the Revox, for 50 mV out of the Troughline, will be (19/66 x 50) = 14.4 mV, which should still be enough, assuming that there is no fault in the Troughline causing abnormally low output.


Cheers,
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Old 6th Apr 2019, 11:28 am   #23
bikerhifinut
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Default Re: Leak Troughline Stereo low output

Synchrodyne wrote this:

"Note that this comment applies only to the Troughline Stereo, which has unbuffered, high impedance stereo outputs. As far as I know, the Troughline, Troughline II and Troughline 3 all have buffered (cathode follower) outputs with the de-emphasis filters on the input side of the cathode followers, and therefore unaffected by output loading. The mono output of the Troughline Stereo is buffered, but on the output side it has a highish impedance attenuator to bring the level down to that of the stereo outputs, and the de-emphasis is done at this point. So it is likely to be affected by the load impedance that it sees."

This is true, however as far as I know, the mono tuners still fed the audio output via the wiper of a 50k potentiometer so this will affect the output resistance although the output does come off the cathode follower.
If I was to use a low Z input amplifier, even with the mono tuner, I'd probably move the output to the top of the carbon track. i.e. off the CF output direct, and leave the pot in parallel. This would also give the dc blocking/coupling capacitor somewhere to dump any excess charge to earth. As it usually is in the form of a "Flying lead" It would be quite possible to insert an extra one I guess and leave the adjustable one in place.
Just a thought. Can anyone see a reason why it wouldnt work?
A.
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Old 6th Apr 2019, 1:41 pm   #24
vidjoman
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Default Re: Leak Troughline Stereo low output

Don't forget that these Leak tuners were expected to be used with the pre-amp that was a part of a 'system' matched by the manufacturer. They were not 'at the time' expected to match any amp you wanted to use it with, especially those of later design where the input levels are now so much higher. As mentioned previously the Leak pre-amp, at the time, had a sensitivity switch and was designed to work with this tuner.
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Old 7th Apr 2019, 12:43 am   #25
Synchrodyne
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Default Re: Leak Troughline Stereo low output

Maybe the answer is to acquire a suitable Leak control unit (Varislope Stereo?) and interpose that between the Troughline and the Linn Kairn. But then there would be the problem of finding a power supply for the Leak control unit. Back in the day, some makers, including Lowther, Jason, Chapman and I think Goodsell and probably others offered standalone power suppliers for power tuners and other items that did not have inbuilt supplies. But I suspect that the survival rate for these had been quite low. Thus building a power supply might be necessary.

Quote:
Originally Posted by bikerhifinut View Post
This is true, however as far as I know, the mono tuners still fed the audio output via the wiper of a 50k potentiometer so this will affect the output resistance although the output does come off the cathode follower.
If I was to use a low Z input amplifier, even with the mono tuner, I'd probably move the output to the top of the carbon track. i.e. off the CF output direct, and leave the pot in parallel. This would also give the dc blocking/coupling capacitor somewhere to dump any excess charge to earth. As it usually is in the form of a "Flying lead" It would be quite possible to insert an extra one I guess and leave the adjustable one in place.
Just a thought. Can anyone see a reason why it wouldnt work?
That seems reasonable. With the 50k pot, the worst case output impedance would be at the halfway point, effectively with 25k in parallel with 25k giving 12.5k. That might not be too problematical in most cases. However, I can recall a time in a previous life when I connected a source with a 10k output impedance via one of those low-cost Radio Shack audio isolating transformers, as I needed to break a ground loop. The bass disappeared almost completely, as the said transformer obviously had very low effective impedance at lower frequencies. The problem was solved by moving up to Sowter 10k bridging transformers, later replaced by a Sescom TR145 in-line transformers. Level and impedance matching is an interesting game!


Cheers,
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Old 8th Apr 2019, 11:44 pm   #26
bikerhifinut
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Default Re: Leak Troughline Stereo low output

Just a thought.
Putting a Leak Preamp (Point one Stereo or varislope stereo) won't solve the impedance issue unfortunately as the output of all the leak preamps is fed via a series of 100K pots, 100k linear balance pot, whose wiper goes to another 100k log Volume pot and thence the output is fed off the wiper of that pot, and all off the anode of an EF86, so lo Z it ain't! Not an issue with a valve power amp with an input Z of 1meg though.
And as the nominal output of a leak preamp would be 125mV to suit the power amps I think there wouldn't be any meaningful gain from it all.

A.
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Old 9th Apr 2019, 12:06 am   #27
Synchrodyne
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Default Re: Leak Troughline Stereo low output

Thanks. I had forgotten that the Leak control units had quite low outputs at highish impedance. So no joy there!

Cheers,
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