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Old 3rd Apr 2019, 5:26 pm   #1
Linnovice
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Default Leak Troughline Stereo low output

Hi all, my current system consists of a Linn Kairn pre amp driving three Linn LK100 active power amps into a pair of Linn Kaber speakers. All sounding very nice. I’ve recently acquired a Leak Troughline Stereo Tuner which has been completely overhauled and sounds stunning.
The problem is that output of the Leak is very low. Around 50 mV I believe. The sensitivity of the tuner inputs of the Kairn is 188mV. This means to get any level of volume in the system the volume has to be cranked to very nearly full (95% according to the Linn scale). This is not a major problem as the quality of the signal is so good. When it comes to recording anything though it’s a disaster. The VU meters are only measuring -18dB at peak readings. Most of the time they are not registering anything. What can I do to increase the output of the Leak?
Any ideas would be very welcome. Mike.
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Old 3rd Apr 2019, 5:42 pm   #2
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Default Re: Leak Troughline Stereo low output

I am surprised that the output is so low. I have a Troughline tuner and never had any problems with connecting it into the "tuner" input to different amps. I wonder if it has a fault? The problem may lie with the stereo decoder. This may have a fault on it, restricting the audio level through to the output.
Alan.

Last edited by Biggles; 3rd Apr 2019 at 5:44 pm. Reason: didn't read the original post properly before answering!
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Old 3rd Apr 2019, 6:19 pm   #3
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Default Re: Leak Troughline Stereo low output

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Originally Posted by Biggles View Post
I am surprised that the output is so low. I have a Troughline tuner and never had any problems with connecting it into the "tuner" input to different amps. I wonder if it has a fault? The problem may lie with the stereo decoder. This may have a fault on it, restricting the audio level through to the output.
Alan.
I agree with Alan, my Troughline mono gives a very adequate output sufficient to give full output on my 500mV sensitivity amplifier. The original Leak paperwork says the output voltage is 1V rms so if you are only getting 50mV there is definitely a problem. I'd investigate the tuner itself. You haven't said if its a stereo or mono version? The mono version has its output fed off the wiper of a 50k potentiometer, there may be a problem there? The actual audio output of the mono versions comes off the triode section on a ECF80 as a cathode follower and the stereo version is obviously off the decoder. If an original decoder I think it used the AF126 types of transistors, I may be wrong there, but that might be an issue if original? But I think those ones didnt get "tin whiskers".
The output is fed off the ECF80 Cathode by a 1uF electrolytic capacitor which may be faulty, but do not assume anything. I'd check the voltages in the circuit against those given in the circuit, these are available with a simple google search online, I won't post any links in case the service info is available via the service on this site.

Andy.
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Old 3rd Apr 2019, 6:21 pm   #4
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Default Re: Leak Troughline Stereo low output

ooops it is a stereo sorry. Anyway it should still push out about a Volt.
A quick test of the decoder, if its original and not been altered is to simply switch to mono and the output will come straight off the ECF80 cathode. If the volume increases then it's the decoder.

A.
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Old 3rd Apr 2019, 6:41 pm   #5
Linnovice
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Default Re: Leak Troughline Stereo low output

Hi, The Leak manual states:

AUDIO OUTPUTS. The two grey screened leads should be plugged into the L and R Tuner input sockets. The output from these leads is much lower than from the Leak Troughline II or Troughline III. The input of the associated audio amplifier should be approximately 50mV with an impedance of 47,000 to 100,000ohms.

Thats is edited but essentially that is the information they give.
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Old 3rd Apr 2019, 8:24 pm   #6
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Default Re: Leak Troughline Stereo low output

That seems very low spec. I wonder if there is a typo in the information and it should be 500mV? Perhaps the restorer has set the output low to meet that spec. Definitely needs more investigation as most tuners I have come across give something like 500mV, enough to drive an aux i/p on any amp.
Alan.
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Old 3rd Apr 2019, 8:39 pm   #7
bikerhifinut
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Default Re: Leak Troughline Stereo low output

Junk the original decoder and fit a proper one.

Or be prepared to accept mono for recordings and switch the turner to mono when the output will come off the ECF80 output triode cathode. I had a look again at the circuit diagram and the output stage for the mono and stereo version of the troughline3 is identical so there must be massive losses in the decoder.

That's it basically. I wouldnt use a booster amp after the output, just get a decent decoder, there are options out there ranging from decent value for money to ridiculously expensive.

Or get a tuner more suited to the job, the troughline has a good reputation for sure but much as I personally like them, there are plenty out there as good. And these days for pennies.

A.
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Old 3rd Apr 2019, 8:51 pm   #8
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Default Re: Leak Troughline Stereo low output

The matching Leak amplifiers around the time that the tuner was current had a switch to change the sensitivity of the tuner input so the lower output was not a problem.
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Old 3rd Apr 2019, 8:56 pm   #9
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Default Re: Leak Troughline Stereo low output

Quote:
Originally Posted by Biggles View Post
That seems very low spec. I wonder if there is a typo in the information and it should be 500mV? Perhaps the restorer has set the output low to meet that spec. Definitely needs more investigation as most tuners I have come across give something like 500mV, enough to drive an aux i/p on any amp.
Alan.
Alan, the official Leak Documentation states "150mV from 75Kc/s deviation i.e. 50 to 80mV from normal signals" (sic).
The only real difference between a troughline 3 and the stereo version, downstream of the detector is the decoder, looking at the circuit diagram.
there's a few bespoke decoder kits out there to bring the troughie up to date and that's the O/P's best bet IMHO.
Also I wonder if there's a bit of an impedance mismatch going on into the OP's tape recorder off the decoder? Shouldnt be a problem directly off the Cathode follower though in mono.

A.
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Old 3rd Apr 2019, 9:22 pm   #10
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Default Re: Leak Troughline Stereo low output

The original fitted decoder is an exercise in being as compact and simple as they could in the day combined with minimal power consumption (c.24V at around 7mA ISTR, derived by dropper from the HT), rather than stunning performance, and the output is both low level and high impedance. I'd look into an external decoder for best results, or a x10 gain block with a dual op-amp and a bit of power- plus/minus 9 to 15V at a few mA- could be put between tuner and amp.
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Old 3rd Apr 2019, 10:26 pm   #11
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Default Re: Leak Troughline Stereo low output

On my Leak Trough Line tuner, I changed the original decoder for one from a circuit I found in one of the various Hi-Fi mags years ago. I think the IC was a CA3090, anyway it worked very well, and still does. If I remember correctly the audio output was about 500mV

The Leak manual mentions 150mV or 50-80mV per channel from the original decoder, but then it only had 3 transistors in it, so you cannot expect much more
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Old 3rd Apr 2019, 10:27 pm   #12
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Default Re: Leak Troughline Stereo low output

That does seem low even for a full deviation signal. If that's the spec, there it is in black and white from the manufacturer. I have never actually tested the output from mine on a scope. 50mV is opening the system up to noise in my opinion. As I previously mentioned, most tuners that have passed through my hands have had stacks of output. Well, you live and learn.
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Old 3rd Apr 2019, 11:06 pm   #13
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Default Re: Leak Troughline Stereo low output

The tuner input on the Linn Kairn has an impedance of 24 k, which is well below the 47 k minimum load specified by Leak for the Troughline Stereo tuner. This would have the effect of further reducing the output level. Approximately, the Troughline Stereo had an output impedance, in the stereo mode, of 47 k, at least below the de-emphasis turnover point. Given that this evidently provided 50 mV (at 30% modulation) into 47 k, then one might expect not much more than 30 mV into 24 k. Not only that, but the de-emphasis curve would be adversely affected, perhaps audibly so.

This reinforces the previously stated desirability of either placing a buffer amplifier between the Troughline and the Kairn, or using a different decoder with buffered outputs and adequate output level. 200 mV at 30% modulation would be a reasonable match to the 188 mV Kairn input sensitivity.

In the valve era, a tuner output of 100 mV at 30% modulation into 47 k or higher was some kind of norm, at least in British practice. Some tuners had variable outputs whose range included that point. Correspondingly amplifier tuner inputs provided were 100 mV sensitivity and 100 k (or higher) impedance. In the solid-state era, there was some upward movement in tuner output levels. By way of an indicative example, the Armstrong 600 series of the early 1970s looked both backwards and forwards; it could be set for an output of 100 mV at 30% modulation or 775 mV at 100% modulation. Also, starting in the 1970s I think, there was a downward trend in “line” input impedances, probably on the (reasonable) assumption that contemporary “line” sources such as tuners would have buffered outputs that were happy with loads from say 10 k upwards. Of course, this created a difficulty when matching old sources with new control units, as this case illustrates.

Even by the valve era norm, the Troughline Stereo output level was on the low side. Leak could get away with this because its control units had tuner inputs that could be switched to cater for this low level. That may have been a legacy from the pre-FM era, when Leak’s own VS AM tuner had an output of 50 mV at 30% modulation. With high quality AM tuners with unbuffered outputs, it was common practice to seriously tap-down the output to avoid the distortion caused when the demodulator AC load materially exceeded its DC load. With integrated radio receivers, the AC load was a known factor, but with tuners that might be connected to a variety of amplifiers, the AC load was a variable, so extra caution was desirable. Of course, the use of a cathode follower buffer would have obviated the problem and allowed a higher output, but British tuner design in that period was severely constrained by the relatively high purchase tax that applied to tuners but not to amplifiers, leading makers to leave out of tuners that which could be included amplifiers, including power supplies and buffer stages. So did the ghosts of constraints of earlier times haunt later practice. Leak was actually quite early in breaking the mould, in that its original Troughline FM tuner had a built-in power supply and a cathode follower output. (Although I think that the Troughline’s reputation was made with the Mk II, which in particular had a three-gang front end with a cascode RF amplifier in place of the two-gang and pentode of the original.) But despite that bold move, as turretslug had said, the Troughline Stereo was very constrained in the decoder department. Evidently there was not enough spare power and/or space available to add a two-transistor (gain stage, say 10 dB or so and emitter follower) buffer in each channel.


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Old 3rd Apr 2019, 11:47 pm   #14
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Default Re: Leak Troughline Stereo low output

I should have added that the Linn Kairn appears to have buffered tape outputs of 1 k output impedance with unity gain from the line inputs. Thus with the buffer, the loading presented by the tape deck line input will not have any effect on the loading seen at the tuner input. But the signal voltage delivered at the tape output will be the same as that which the tuner provides at the tuner input, in this case estimated at around 30 mV at 30% modulation. That I think would be on the low side for some tape decks, although it might suit some DIN-type inputs.



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Old 4th Apr 2019, 7:31 am   #15
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Default Re: Leak Troughline Stereo low output

The original Troughline stereo tuner, and the modified mono tuners, used a Mullard/Philips decoder. Also different was the front end that was changed from ECC85 to ECC88 to improve the gain.
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Old 4th Apr 2019, 8:09 am   #16
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Default Re: Leak Troughline Stereo low output

Hi, thank you all for your responses and taking the time.

I have a small collection of tape decks. This includes a couple of Studers, Revox A77s, PR99 and Teacs (3340 & 3440). The signal coming from the Kairn is so low as to be impractical to record. It’s there on headphones but the meters hardly deflect. 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.

It also has a down side. I don’t think the Kairn likes being fed from a patchbay. Because of the amount of cables/connections I upped the patchbay from 24 channels to 48 yesterday afternoon. On startup there was a massive feedback that promptly blew both tape loops on the Kairn.

Do you ever get one of those days
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Old 4th Apr 2019, 11:12 am   #17
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Default Re: Leak Troughline Stereo low output

Hi Linnovice,
Synchrodynes very comprehensive answer and explanation pretty much says it all for you. He's joined all the dots up and you should be able to get sorted out.

For now, until you get a proper decoder on it, and frankly I think that's the only way to get decent stereo performance, you can simply bypass the original decoder and the circuit diagram indicates that simply switching to mono should achieve a low output resistance off the cathode follower and a healthy output voltage and at least you'll get the good sound quality that the troughline is capable of.

If it was me, I'd keep the Troughie as a very nice mono valve tuner and a thing of beauty in itself, and lay my hands on a modern(ish) FM stereo Tuner of which there are many excellent examples and these days literally for a few quid. There were loads of really good japanese ones in the 70s and 80s and as long as you have a good strong FM signal from the aerial, something like my own Arcam Delta280 will perform very well. It's not so good when the signal strength is a bit borderline I have found, whereas the japanese stuff seems to be very good in this department.
The only reason I wouldn't go down the aftermarket decoder route is that I personally couldn't justify the expense of either building one or buying one in as I could buy an entire tuner for the same or less.

It's given me food for thought about my own Troughline stereo that's on the roundtuit pile.

A.
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Old 4th Apr 2019, 2:38 pm   #18
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Default Re: Leak Troughline Stereo low output

Hi A. I take your point and I don’t think I’m being a Luddite. I already have a Teac FM/DAB tuner, Sansui TU317, Linn Kudos and a Leak Stereofetic. I really do believe, output level to one side, that the dynamic range, soundstage and overall ‘feel’ of the Troughline is superior. The nearest is the Stereofetic. If I could capture that ‘sound’ on tape I’d be a very happy man.

Right now though, I need to sort out the tape section of my Kairn as at present I can’t record anything. But, and you’d never believe it, Linn don’t want to know. They’ll sell me a new DS unit for 2.5£K though. That will do everything I don’t need but nothing I want. It will also save me £100’s on interconnect cables so I’m told, that’s nice to know.
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Old 4th Apr 2019, 5:22 pm   #19
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Default Re: Leak Troughline Stereo low output

Quote:
Originally Posted by Linnovice View Post
Right now though, I need to sort out the tape section of my Kairn as at present I can’t record anything. But, and you’d never believe it, Linn don’t want to know. They’ll sell me a new DS unit for 2.5£K though.
Not unusual with Linn, sad to say.
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Old 4th Apr 2019, 9:28 pm   #20
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Default Re: Leak Troughline Stereo low output

I think we have to remember that the Mullard/Philips stereo decoder was the main one available at the time, the circuit being supplied to manufacturers who wanted to use it. Leak just bought in the ready made unit and fitted it to the tuner. It worked very well, at the time, before other decoders became available. Remember that IC’s were not available at the time. Modern decoders are generally better but so are modern motor cars compared to the 1950’s.
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