Side Projects

[Mike H]

Side Projects
Evenin' all. Just a quick (ish) update re couple of latest doings.


More Betterer Bluetooth and Pot-in-a-Box (aka 'DIN Amp' or pre-amp)

Don't worry Kubelik is in progress!


#1.
Bluetooth recap -
I have been using this 'Bluetooth adaptor' which has a daugter board as the ubiquitous (apparently) KRC-86B...
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KRC-86B Blutooth Adaptor.jpg (37.7 KiB) Viewed 14669 times

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[latterly] I detected an artifact which I believe to be, what I call, 'capacitor mush'. There are DC blocking caps in the signal paths somewhere, but I haven't yet identified exactly where they are, but they are undoubtedly SMD. The whole thing is too involved anyway, there's a multiplexer chip (i.e., a solid state stereo selector switch), and at least one dual op-amp. (And 2 other 8-DIL chips with no markings.) The idea is that there is another, 'default', stereo audio input, but when the receiver establishes a Bluetooth connection, it switches over to Bluetooth audio. The module is intended for Bluetooth speakers, apparently. I don't need that much complexity and so then I found a different board, that just has audio straight out of the chip, via a couple of easily accessible caps.

So the final plan was to pull the KRC-86 off the motherboard (which has a 5V regulator and connectors) and basically replace it with 'Bluetooth 4.1 5.0 Audio Receiver board' ... the chip is one of the JL AC19AP-1XXXX-25A4 series ...
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Bluetooth 4_1 5_0 Audio Receiver board-2.jpg (98.5 KiB) Viewed 14669 times

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Actually replacing it was going to be awkward but it can simply be connected - I found a handy plastic box and construction commenced -
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New Bluetooth-001.jpg (82.21 KiB) Viewed 14669 times

[Mike H]

Drilling holes ...
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New Bluetooth-002.jpg (52.79 KiB) Viewed 14667 times

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Fitted sockets ... supply is from an external 12VDC adaptor.
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New Bluetooth-003.jpg (79.93 KiB) Viewed 14667 times

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Wired up - I made a mistake and got the supply wires the wrong way round. Oh no I've buggered it! It didn't light up so I immediately disconnected it. The 7805 reg was suddenly very hot. Rewired it properly and it still worked! Yay! Hooray for short-circuit protection. But implies there's a reversed diode on the board somewhere or maybe even in the chip!
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New Bluetooth-004.jpg (81.83 KiB) Viewed 14667 times

[Mike H]

Working - hole drilled for blue LED on the board -
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Bluetooth module.jpg (65.6 KiB) Viewed 14667 times

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First Impressions
Big improvement over the KRC-86B. Using my own interconnects probably helps as well (instrument cable and gold-plated phonos), instead of the 3.5mm jack to phono lead (eBay, not tat tho). Nearly as good as a DAC. Will do the do for most everyday purposes, so I am very satisfied with it. Was worth doing. Gets a green tick

[Mike H]

#2.
DIN Amp (pre-amp)
Sometimes I'm in need of a bit more gain, hence this idea. The following is the original DIN amp schematic, from the Akai GX-370D reel-to-reel service manual - so-called because it received input from a 5-pin DIN socket. (A previous owner replaced this with 4 phono sockets, but which are quite ropey and I need to do something about them.)
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Akai DIN Amp.jpg (48.95 KiB) Viewed 14666 times

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In the original, the DIN amp goes out to a 100k pot, thence is mixed with the output of the microphone pre-amp (for dubbing purposes) - which also has it's own output level pot - the mixing is via two 47k resistors. The maximum level from either is therefore immediately reduced by half. There's some complicated switching involved as well, but eventually, the signal returns to the board for the record amplifier section, which outputs to the tape recording head. By virtue of the ratio of R14 to R15, the DIN amp has a gain of slightly over 20 dB (X10). This is knocked down to an actual 20 dB, or somewhat less, by the output load (100k pot, in parallel with the divider 47k resistors).

'X' number of years ago I made this board below, but pretty sure I never used it. Emitter followers were added to the outputs. Then some while later again, I decided to change out all the Maplin Min Res (1% metal film) for carbon film resistors. Still never used it! Whatever it was I was going to use it with, never got built. (Twice!)
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DIN Amp build-004.jpg (115.75 KiB) Viewed 14666 times

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This time I wasn't overly sure about the quality of the 'lytics so changed them out from my latest 'stocks' - I think they're now a combination of Panasonic and Jensen. Resistors are Truohm carbon film (Rapid Electronics 0.5W Carbon Film resistor kit) and Takman carbon film (HiFi Collective). Transistors are (and always were) BC548 and BC549.
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DIN Amp build-006.jpg (150.06 KiB) Viewed 14666 times

[Mike H]

Final schematic -
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Akai DIN Amp Clone version 2.gif (56.34 KiB) Viewed 14665 times

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Construction, using cute wee box -
Not shown in schematic, simple switched inputs (choice of two).
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DIN Amp build-001.jpg (85.53 KiB) Viewed 14665 times

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DIN Amp build-002.jpg (93.4 KiB) Viewed 14665 times

[Mike H]

Continues...
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DIN Amp build-005.jpg (108.71 KiB) Viewed 14665 times

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[Mike H]

Testing the board -
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DIN Amp build-007.jpg (81.26 KiB) Viewed 14665 times

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DIN Amp build-008.jpg (78.67 KiB) Viewed 14665 times

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DIN Amp build-009.jpg (76.19 KiB) Viewed 14665 times

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Yep! We'll go with that!

[Mike H]

Construction continues...
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DIN Amp build-010.jpg (102.95 KiB) Viewed 14665 times

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DIN Amp build-011.jpg (114.09 KiB) Viewed 14665 times

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[Mike H]

Connected up and working -
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DIN Amp build-012.jpg (68.54 KiB) Viewed 14663 times

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DIN Amp build-014.jpg (87.42 KiB) Viewed 14663 times

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I am using a bench power supply to run it - it needs 20 - 25V DC.


First Impressions
Wow it has a very clear and open sound. Nice musical details. (Do interconnects need to be 'driven'?) May be due to the pot (Bourns 100k carbon linear from RS).
Definately worth doing, and gets a
Happy with that.

Appendix
After a while I noticed the capacitors C3 were 100 pF not 1,000 pF (1 nF), and I vaguely remember thinking at the time 1 nF was too much and will roll off the treble, not understanding at the time what they're for. So then I replaced with mylar film 1 nF; mylar caps were 'in fashion' back in the '70's and '80's (the Akai tape recorder has lots of them), these ones came from Crickewood Electronics and AFAIK are NOS or close to it. Now to my ears mylar caps can sound slightly 'edgy', but in this case it's not noticeable. Or, if they are, they do help focus the treble a weeny bit.

ENDS

Kubelik next.....

Laters...

[Ray P]

Kubelik next.....

Waiting with interest.

In the meantime, for your DIN amp (glad it doesn't make a din BTW), I have a used 10K Alps blue potentiometer you can have that might that might be a small improvement. You'll need to unsolder it from a PCB but for a man of your abilities that shouldn't be a problem! Let me know if you want to try it and I'll pop it in the post.

[Mike H]

Hi Ray, I've got a few, and have tried, Alps blues, and I don't think I like them. Or at least I didn't - unfortunately due to design of the case this has to have a splined shaft and a smaller boss (smaller than 10mm / 3/8" )

I remember now - the Alps blue seemed to act like an 'expander', that is, 'suppress' quiet sounds, with the effect of making loud sounds sound louder.

[Ray P]

Fairy 'nuff Mike.

[Tony Moore]

I've had some Ebay/Aliexpress "Alps Blue" that I think are probably fake as they're so cheap. However they sound better than Alps Blue to me.

Might be worth a try Mike? They are dirt cheap. If I can dig one out I'll send it over to yo to try.

[Ant]

One i like which is a bit dearer than an alps blue from hfc, is a tkd 2cp601
These are very small, shaft is quite short too and not splined, but a nice one for a relatively reasonable price

[simon]

I have two or three of those TKDs too Ant, seem pretty good without going to silly money.

They're a bit fiddly to wire up as they're small, as you say Ant. I drafted a small PCB with solder pads for wires which is much easier to use than splaying the pins and trying to solder wires to them.