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1970s Design Indulgence

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Post Options Post Options   Thanks (1) Thanks(1)   Quote patientot Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 5:17pm
Here's what the specs on my 1995 JVC cassette deck say.




Edited by patientot - 24 Nov 2022 at 5:17pm
Reflex M + PSU-1 used with AT150-40ML, AT VM95ML, Stanton 680mkII + Ogura, and Shure M35X cartridges.
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The EQ Kid

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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 7:25pm
From the Nortronics Silver Catalogue:

Regarding bass:

"The longest wavelength the play head can respond to is equal
to the mean of the window and pole lengths. High tape
speeds such as 15 ips or 30 ips on professional recorders
pose severe problems to play head designs since the longest
wavelengths may run up to 1-inch, requiring open head faces
and very long poles. These are more expensive and also require
external magnetic shielding against hum pickup from
motors and transformers."

Which basically means that if you're using 1/4" tape, your machine (if a good 4 track) has premium heads and its best bass is at 7.5 ips.

But, at 3.75 ips it will still render music reasonably.

And as cassette head outputs are ridiculously low, cassette deck users should stick to the deck's line out.

Therefore, this preamp/line-stage tape input will be for 1/4" tape deck tape head outputs, at 7.5 and 3.75 ips.

Simulated response:

preamp tape head responses

Simulation circuit:

4 transistor &quot;Dinsdale&quot; in tape guise

R19 = 75k in series with 51k for 3.75 ips, and 75k alone for 7.5 ips

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 7:31pm
Originally posted by patientot patientot wrote:

Here's what the specs on my 1995 JVC cassette deck say.



Best connected to the rec out and 3rd head input on block diagram.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 7:49pm
Well, best try this "universal circuit" on something. As I don't have a tape recorder, the next most stressful configuration to prove (or disprove) it works, is the phono stage config.

I have some waste power amp boards that will accommodate all the components one way or another, and a PSU1 will supply the 22.5V via an RC dropper.

Just need a chunk of plywood to mount the boards and connectors on.

Let's hope I haven't screwed up anywhere (I usually do...) Embarrassed

4 transistor &amp;quot;Dinsdale&amp;quot; in RIAA guise

EDIT: R5 and C10 will be 1k5 and 220pF


Edited by Graham Slee - 24 Nov 2022 at 7:50pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 2022 at 12:01pm
Many years ago, part of my day job was to look after several Tandberg "mono" reel-reel machines. I say "mono" but the heads were two channel, but one half wasn't used, and instead its output was taken to a socket simply named "tape head output". The replay preamp (for the half of the head in use) was the "Dinsdale" two transistor stage and very similar to the circuit being developed here, except for the extra two (output buffer) transistors. The reason I know this, is because I took one apart to find out.

Now, the faults with the two transistor circuit are many. For a start it doesn't do a really good transient response, but that's at really high frequencies, but tape doesn't do really high frequencies because of the head gap and also it's inductance, so it mattered not.

But most folk my age listened using the two transistor stages without knowing, and most I talk to were happy enough with the sound, and in some ways far more happy than with modern circuitry. The other great thing is it can't be "chip rolled" (hooray!)

And who cares that it can't do 120dB S/N? Except perhaps the specification nerds, but what would they know about music, and emotion? Obviously nothing! Reminds me of the Blanks on World's End... LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 2022 at 1:54pm
So this is the test board utilising a scrap power amp board from the time before last, and the two arrows are where the different NFB go across which determines what function it performs.

Does it work? No idea! Got to test it yet. Wink

4 transistor &quot;Dinsdale&quot; test PCB

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 25 Nov 2022 at 4:58pm
First test: DC

Using a 1k2 dropper and 470uF decoupler, the supply voltage using the PSU1 (24V regulated output) is 22.6V, which is close enough to the circuit diagram value of 22.5V.

The DC output should be around 13V, but started at 16V and now is 14.4V, indicating initial capacitor leakage current, which will, given time, decay to a minimum, and will hopefully reach around 13V.

13V allows an r.m.s. output swing of 6.3V, which should be adequate for signal headroom.

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