Low noise valve tube circuit ideas for cassette deck

[hbc]

Hi,

I am making for my own amusement a valve playback cassette deck, I have got a circuit working, loosely along the lines of a mm phono stage but it is a bit noisy, problem is, it seems that the output of the head is more MC voltage, in the sub millivolt range, but the input z of the denon deck I am experimenting on is about 27k resistor iirc. I have tried making a hampton type head amp, and put it in front of my existing amp, and reduced the gain, and the noise floor changed bud did not get much better.

Dave builds a valve cassette deck. - Tapeheads Tape, Audio and Music Forums

I have morgan jones and the sound of silence, books. I have looked at reel to reel, but I suspect that the heads put out more volts, being bigger and faster tape speed etc.

I may be impossible, and am not adverse to sneaking a few jfets in if I must.......

so if anyone has any helpful pointers / links that would be appreciated.

thanks

 

[Eli Duttman]

Other than a different equalization curve, the problem is identical to that of phono. A LOMC cart. "remedy" is indicated, if the playback head is producing a μV. level signal.

As you're flexible about hybrid circuitry, I suggest you investigate the late Allen Wright's hybrid cascode. "Surf" around the site and you'll find out how to order the channel to channel matched pair of JFETs needed.

 

[wa2ise]

You could make use of the "Tape head" input seen on older integrated stereo amps like the SCA35.

 

[Eli Duttman]

You could make use of the "Tape head" input seen on older integrated stereo amps like the SCA35.

Not quite! For starters, that EQ is for 7.5 IPS, not the 1.875 IPS cassette decks use. The OP has already pointed out that the signal produced by the head is insufficient to drive "typical" MM level circuitry.

 

[hbc]

Hi, thanks,

I think part of my problem may be that mc need a fairly low input z whilst the cassette head needs higher z and this resistor noise is not helping. I have some hopefuly not fake 2sk170s, and I am trying to make them work at the.moment. I did try a hi gm triode some ecc88, and pc900, but unfortunately the grid current is a bit high For direct connection to the head.

 

[Eli Duttman]

How high a resistance do you need, to properly load the playback head?

Vishay bulk metal foil parts at higher resistance values are hideously expensive. Perhaps you can source very quiet Caddock TF020 series parts, in a suitable value. The TF020s are expensive, but not nearly as dear as the Vishay parts.

 

[Wavebourn]

1. Microphone transformer
2. 6С17К-В tube

 

[BinaryMike]

Can you get your hands on a few schematics for good cassette decks made back in the days when simple bipolar transistor preamps did the job? Examples like this should help point the way.

 

[kevinkr]

How high a resistance do you need, to properly load the playback head?

Vishay bulk metal foil parts at higher resistance values are hideously expensive. Perhaps you can source very quiet Caddock TF020 series parts, in a suitable value. The TF020s are expensive, but not nearly as dear as the Vishay parts.

Johnson noise is constant regardless of construction, in terms of excess noise once you reach wirewound or good metal film levels of performance small reductions in the remaining excess noise come at very high cost. If a good metal film isn't acceptable hideously expensive bulk foils aren't going to make much difference.

Transformers are an option if the head impedance is a few hundred ohms or less, a cascode with 6S3P or D3A might also be an option.

Cassette playback without Dolby or ANRS noise reduction systems is very noisy, the best SNR achieved back in the day without noise reduction was not much better than 50dB.. My first, a Hitachi I think achieved 48dB typical A wtd, and it cost over $250 in 1973. (No Dolby)

 

[hbc]

Hi,

I think I may have been confused re input z, The Denon uses a 56k to load the head. I had considered a transformer, but I would rather not due to cost, today, at this stage.

I have successfully now built this:

A simple noise-reduction circuit. - Tapeheads Tape, Audio and Music Forums

jfet amp, and to be honest it was a pleasure, just worked, I dropped it into the Denon, in front of my tube circuit, and my circuit is now subjectively quieter than the decks own amplifier. it was almost too easy compared to all the tube wrestling I have been doing over the last week or so.

I am going to carry on my low noise tube experiments, but now have a benchmark to compare them to.... and if it all goes wrong.... I have some where to go.

I have ordered my self some 6s3ps to play with, I am sure I have some EC86s some where...

Another thing I now realise i am confused about is how the 56k resistor noise interacts with the inductance / DCR of the head. I guess I will have to measure the inductance, and see what it is.

Hi Wavebourn, forgive my ignorance but when I look up 6C17K-B I come up with RF power triode....

I have a few transistor cassette decks, and to be honest, how simple they are helped nudge me in to starting this project. most only use 2 or 3 transistors before it disappears in to dolby (TM) land.

thanks for the inputs.

 

[piano3]

1. Microphone transformer
2. 6С17К-В tube

Definitely agree!

 

[wa2ise]

Question: Would a tube preamp make sense for tapes with Dolby? Does the Dolby get ruined by the "tube sound" the tube preamp would add to the audio before the Dolby deemphasis? Or would the Dolby deemphasis ruin the tube sound? Or do they play nice with each other?

 

[Hearinspace]

I just finished a first working build of an MC phono stage based on Rod Coleman's shunt cascode circuit. I've been listening to it for a few days now and it is very good right out of the chute with lots of room for fine tuning. The load resistors (one on the jFet first stage and one on the triode second stage) determine the voltage gain which makes it easily adjustable. No second thoughts about recommending it. Mine is a pile of wire spaghetti and it's still quiet. The filter will be different of course but my guess is that Rod would be happy to answer questions about adapting the eq circuit for tape if you post.

A search of "Shunt Cascode" by author netted these.

hope this helps some

 

[Wavebourn]

Hi Wavebourn, forgive my ignorance but when I look up 6C17K-B I come up with RF power triode....

Don't worry. It is "Power triode" for gigagertz frequencies. For audio frequencies it is high-mu, high-gm, low noise and zero microphonic miniature planar triode. ;-)

The best tube I ever saw for phono stages and microphone preamps. Your tape deck definitely has heads with very low inductance and output, so the main concern of the input stage is noise. So, a low noise tube and a transformer is the answer. Otherwise, I would suggest you some medium power HF transistor, because it has very low base resistance that means very low noises. Once in a laboratory of an institute I was searching for a low noise transistor for a mic preamp... It was in late 70'Th... All so called "low noise" transistors did not satisfy me. I tried a KT626 laying on the table, and was amazed.

Specs for "high power" do not make devices bad for "low power", similarly like specs for "high frequency" don't make them bad for low frequencies.

 

[hbc]

Hi,

aha, it is not always easy to judge scale.... The RF triodes i have seen were all 50 - 100 watts forced air cooling, but looked quite similar at first glance..... Em presumably they have some kind of socket, does it have a name?

That looks like a nice phono stage, but I am trying not to use bipolar transistors for religious reasons on this project.. but very much enjoy unexpected synergistic results that can be had from hybrid circuits.

re dolby, already the cleanness of the sound works ok with dolby encoded cassettes. once the eq amp is good enough I will be on to the valve dolby b decoder. I am going to base this on the discrete standard circuit probably. I will be keeping the solid state side chain and the "jfet" but attempting to replace any audiio amplifier with tube circuits..................

 

[piano3]

No available socket that I've ever seen, you can just solder directly; it's quite easy.

 

[Wavebourn]

No available socket that I've ever seen, you can just solder directly; it's quite easy.

I bought a bunch of octal plugs on ePay, going to solder 6S17K tubes to them, to select pairs.

They have 18 micron grids with 8 micron thick wires, so with such microscopic sizes they have wide spread of parameters. I need pairs, so they have identical grid currents, to avoid magnetizing input transformer core.

 

[piano3]

I'm not sure I understand what you mean Anatoliy; are you using a differential pair but with the cathodes connected directly to ground?

 

[Wavebourn]

I'm not sure I understand what you mean Anatoliy; are you using a differential pair but with the cathodes connected directly to ground?

Yes. Not a LTP, but a pair. ;-)

For a LTP I would need too many very clean 6.3V regulators.

 

[cbdb]

Why, a cassette has a S/N ratio of 45 db.