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1970s Design Indulgence
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Points: 16298 		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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Points: 16298 		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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Points: 1525 		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
SL-1200 MK7 (modified) + Reflex M + PSU-1 used with AT150-40ML, AT VM95ML, Stanton 680mkII + Ogura, and Shure M35X cartridges.
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Points: 16298 		Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 4:41pm
I think we'd best forget about cassette tape head preamplification. Their outputs are in the region 0.05 to 0.1mV at 1kHz, and our input stage is being pushed at 52dB 1kHz gain (x400). This input might be good for reel-reel where tape head outputs are nearer 1mV, and a 400mV output in a nominal 500mV preamp output at -2dB is hardly going to be noticed.
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Points: 16298 		Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 24 Nov 2022 at 4:11pm
Tape head preamp design

Starting here are a collection of facts we will require along the way.

The frequencies of constant velocity (my borrowed term from cartridge EQ) 

Reel to Reel

7.5 ips 50Hz - 3180Hz
3.75 ips 50Hz - 1800Hz

Cassette

Fe02 50Hz - 1326Hz
Cr02 50Hz - 2274Hz

Now to hunt for head output information...
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Points: 16298 		Post Options Post Options   Thanks (2) Thanks(2)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 2022 at 6:40pm
Originally posted by peterb peterb wrote:

I have been catching up on this thread and read with interest the section on signal switching. 
My recollection of a couple of 60/70's DIY Preamps I made were that the switches were always a problem after a year or two, nearly always getting noisy and needing Switch Cleaner. Even what were supposed to be enclosed push button switches seem to suffer from this.
My current Preamp uses sealed miniature relays and is still 'quite' after 3 years of use. Perhaps not very 70's but I am sure there must have been some designs that used them. They can certainly make the wiring simpler as it can all happen on a PCB.
Sorry if this is a bit of a digression. Smile

I can understand that with "wavechange" switches. Those tend to be attacked by soldering fluxes, as the stationary contact is an extension of the PCB pin, and are difficult to seal.

The proposed rotary switch solder pins are also an extension of the switch stationary contact, but there should be more distance between soldered joint and contact. I would prefer gold plated contacts as gold is inert. The rotary switch wiping contacts should also help.

With relays the minimum contact current becomes a problem with very small signals, and it is hard to find any today bettering 10uA. In push button channel selection desks we ended up using DIL reed relays, with over current protection to prevent burn out. Performance was excellent, but try obtaining them today.
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Points: 16298 		Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 23 Nov 2022 at 6:28pm
Originally posted by Sylvain Sylvain wrote:

yrGraham may be ihtting on a topic that may have me and many convinced that Integrated design are flawed intrinsically. Passive design is or was the '' way to go'' because the design constraints of Integrating the electrical and mechanical and Power Supply TECHNICAL intricate issues and  are too much to retain the natural tonal musical flow. 

The problem with a passive "preamp" is the attenuation forms a high series resistance, which when loaded by this power amplifier, will cut the high frequencies.

The musical flow will be retained if the circuits have a good equal slew-rate, wide frequency response and good phase flatness, and if the source to next stage impedances are better than 10:1.

It will also be noted that the power supply difficulties have been ironed out in the power amplifier sections and lessons learnt.
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