Project 1960s Rediffusion Reditune TP48A

As a parallel task to all this, and as a fallback, had the Reditune projects failed, I bought and installed a very discreet, modern Background system. It basically looks like a power brick and is Bluetooth, and is hidden out of the way. It has a speaker out and one input. Until recently I had downloaded all manner of period background music and was feeding it to this unit. Now the Reditune can take over.

Good thing about the modern system is I installed 3-speakers, one out in the store, one in the top part of the museum and the other in the lower part. The reditune at present just has the one large cabinet. If I digitise the Reditune carts, I can share the load and alternate between the two systems.

Posted by: @cathovisor

are you planning to digitise the tapes for posterity?

Yes, I would like to, but need to figure how and what with. Although the carts have survived in far from ideal condition for over 50-years, that cannot be relied upon to be the case, especially I start using them more and more. Also, the pinch roller support in the carts are showing signs of fatigue. This is my next task, I've been scouring the internet for possible pinch roller replacements and how to come up with a replacement support for them.

Posted by: @crustytv

Yes, I would like to, but need to figure how and what with.

In theory, a lead connected across the track of the volume control in the TP48 will give you a suitable line-level feed into a sound card. Most sound cards seem to be USB devices these days: my trusty old workhorse (well, it was until the HDD died...) was a Windows 2000 machine with an M-Audio "Audiophile 2496" PCI sound card and a copy of Sound Forge to manage the ingest and edit process. That sound card is easily the best I have ever heard - the 2496 refers to 24-bit, 96kHz sampling. Normally I sample at 48kHz (the broadcast standard) but use FFMPEG to resample to 44.1kHz if making CDs of recordings.

There is of course the freeware Audacity for ingest and edit but I have to confess I never found it as intuitive as Sound Forge.

Currently, enjoying period background music in the museum, and I've completed the display area for it.

@crustytv I enjoyed the YT video you've just put up containing twenty minutes of music whilst searching for (a) the charger for my LED work lamp (it's specific to the lamp and the lamp blacked out whilst I was in the loft 🤬 ) and (b) locating the 750mL tin of red Hammerite for a near-future job.

Glad you liked it, although the audio quality was poor due to it being just the phone on a tripod a few feet away, it was so much better in reality. I'll certainly take your advice, and produce some better recordings in due course.

Just listening to the tunes and watching the clock on YouTube - it is very relaxing and letting me escape from the world (this is a bad weekend for me). We *have* to get a buffered output from the TP48 into your PC to record this stuff and similarly, the TP80 when you get that going again. It really shouldn't be too hard to engineer something involving a TL071 or TL072 - I made a buffer board up for my Portogram record player to replace the cack-handed and amateurish method they used for a tape recorder feed. On the TP48 we could send the feed out of the unused pin of the DIN socket used for the gram input - depending on how they wired the DIN socket of course. In theory, if it's treated as an 'amplifier' it'd be pins 2 and 3, with the signal on 3: but if wired like a 'tape recorder', it'd be 1 and 2!.

Oh, why didn't they just make all the audio leads cross-overs!

If you recall, the original 20K Log volume pot was centre tapped, and was in-circuit with the other 20K to provide tone, as David advised, utilising the Fletcher-Munson effect. No centre-tapped log pots were available, so I just installed two brand-new, non centre-tapped 22K log pots, accepting tone control would not function. As this is a background music system, it would likely not matter.

Being a little bored this afternoon, I've been giving some thought to the inoperative tone (loudness) control.

Looking online, I found a 25K (not bought it yet) linear centre-tapped pot. What If I converted the linear pot to log, looking on some audio sites, others have attempted similar conversions with success. Others felt it was not a good idea, as it would load additional current into the wiper. The rule of thumb appears to be 1/5th of the pot value from centre pin to ground, to emulate an audio taper.

Thoughts?

@crustytv Try it - you've got nothing to lose? 

I'm not sure that it would work too well, for a log pot 50% rotation does not correspond to the half resistance point, I think that it's more like one sixth resistance.

i.e. For a 20k pot the cap is across 3.3k (20k/6) to ground and the cut-off frequency will not vary from 50% rotation down to zero (assuming the load on the wiper is negligable).

With a loaded linear pot the cut-off will be a maximum when the wiper is at the tap and decrease as the wiper turns towards zero

e.g With a loaded 25k linear pot at say 25% rotation you would have 12.5k, cap to ground, 6.25k to wiper and 6.25//5k to ground which doesn't 'feel' right at all.

Jim

Hi Jim, thanks for your input, it certainly is a minefield of debate on various audio groups. I seemed to find all manner of opinions, and it was all very confusing, to me anyway. After all, I'm not really noticing any loss of bass at a lower volume level. I think I'll just leave well alone. It was just a thought to idle away Monday afternoon boredom, while I await parts to arrive so I can continue with the TP80.

The enormous (and frighteningly expensive) Decca "Kensington" from c. 1947 didn't have a conventional volume pot but a six-position switched attenuator, each position having its own frequency-shaping network. I say you locate a single pole, twelve way rotary switch and build a log "pot" out of it - you could then tap off at the sixth position!

Thanks for the suggestion, Mike, that sounds rather interesting. I've no real burning desire to pursue either options, as mentioned above, it was just a Monday afternoon musing. 👍 

Admin Note:

David (ancientengineer), has been responding to the site auto generated, e-mails, instead of replying in the thread. These e-mails are sent to a member when a thread, they are subscribed to, receives an update (post). Not sure if David realises his responses are not making it to the thread, but into a black-hole e-mail box.

I will post his latest e-mail response, and hope David reads this, and in future replies in  the threads, not to the e-mails as nobody but I ever sees them.

Posted by: @ancientengineer

I hate to say this,
but it is after all only a background music player, from the 1960's. What emanated from it was what we rather tongue-in -cheek referred to as "Lift-music"!
The distinct frequency range made it what it is, Chris is intending to use it in a museum situation, to exactly re-create the nostalgic sound! Great.
It's a "cycles-per-second era", not Hertz....

Although somewhat academic, in the late 1970's some Japanese amplifiers managed to produce contour compensation without any taps required on the volume pot. Interestingly their solution required a linear law track with a compensation network to provide the necessary logarithmic change together with switchable compensation.

Rich