Beginner, simple circuit question

barbaroja · Oct 17, 2019

Hi. I am facing this circuit before a drive coil in a record cutter.

Any Ideas on why they added this? What is its function? It seems a basic RC circuit, but due to the particularities of cutterheads and their resonances, might be doing something more

Thanks

rackmonkey · Oct 17, 2019

I don’t know anything about cutterheads, but the cutoff frequency of the parallel RC network is right at 20kHz. May simply be a filter for frequencies above the audio band that could cause undesirable resonance that could lead to oscillation.

Just a theory though. Someone here will know, I’m sure.

Bo Deadly · Oct 17, 2019

The parallel RC filter is low impedance at high frequencies. So it allows HF to pass and LF see 4 ohms.

barbaroja · Oct 17, 2019

Alright. So might be a way to "flatten the impedance curve to some extent.

Thanks

PRR · Oct 17, 2019

> Any Ideas on why they added this?

Depends entirely on the cutterhead. Some work best with direct connection to low-Z amplifier. Others work best with some series impedance.

The drive to the cutterhead is low bass and mids but HIGH at the top of the audio band. So it makes sense that the cap provides maximum drive at the top of the band.

Use the network as specified by the cutter designer, until you have 40 years experience(!) using, calibrating, and modifying cutter systems.

rmburrow · Oct 17, 2019

Which cutterhead are you using this circuit with? Westrex? Grampian? Presto 1-D is a 500 ohm cutter head. If you are cutting using RIAA pre-emphasis, there is a LOT of HF component going to the cutter head. I assume the amplifier driving the cutterhead has a minimum of 50 watts clean audio...and use a good limiter to grab "spikes" which can cause damage...

Bo Deadly · Oct 17, 2019

Note that the impedance of a conventional modern tantalum capacitor can be quite high at low frequencies. Even at 1kHz it could easily exceed the 4 ohms of the resistor.

Maybe the "non polar" one in this circuit is special somehow. Although I've never heard of a "non polar" or bipolar tantalum cap.

barbaroja · Oct 17, 2019

rmburrow said:
Which cutterhead are you using this circuit with? Westrex? Grampian? Presto 1-D is a 500 ohm cutter head. If you are cutting using RIAA pre-emphasis, there is a LOT of HF component going to the cutter head. I assume the amplifier driving the cutterhead has a minimum of 50 watts clean audio...and use a good limiter to grab "spikes" which can cause damage...

Westrex 3DIIA with solid state amps from the same brand.

barbaroja · Oct 18, 2019

PRR said:
> Any Ideas on why they added this?

Depends entirely on the cutterhead. Some work best with direct connection to low-Z amplifier. Others work best with some series impedance.

The drive to the cutterhead is low bass and mids but HIGH at the top of the audio band. So it makes sense that the cap provides maximum drive at the top of the band.

Use the network as specified by the cutter designer, until you have 40 years experience(!) using, calibrating, and modifying cutter systems.

I did not say I was modifying it. Just want to understand its workings. Now we have more resources and knowledge than the time when they created those. We are very likely to be able to understand them quicker.

Gold · Oct 18, 2019

It looks like it would be in the feedback network to tame one of the three resonances in a Westrex IIID. If that part of the circuit is on a removable card in the 1700 rack, it was factory matched to the specific head. If the head was rebuilt the EQ card will needs to be tuned to the head.

PRR · Oct 18, 2019

squarewave said:
Note that the impedance of a conventional modern tantalum capacitor can be quite high at low frequencies. Even at 1kHz it could easily exceed the 4 ohms of the resistor.

It should be what it says on the label. 2uFd at 1kHz is 80 Ohms. (Way more than 4r.) Any stray parasitic resistance in leads or stuffing will be sub-Ohm (else it would suck at 20kHz).

Agree with Gold. They used conventional techniques to compensate the several tilts and resonances of the head, and were left with something in the area of 20kHz (2uFd+4r) which was best trimmed with this network. Changing 2uFd to 3uFd or 1.5uFd is likely to have less effect on frequency (because Resonance) so the effect may shift 16kHz or 25kHz, no big deal. With a shift in peak/dip height/depth as well. And if they were paired at the factory, are probably as good as it gets. Westrex were not the most musical folks in the world but they were brilliant technicians.

Gold · Oct 18, 2019

From memory the three resonances in a Westrex IIID are: the primary resonance around 1K, the secondary resonance is about 13K and the tertiary resonance is around 20K. Take that with a grain of salt. I'm pretty sure there are three resonances but unsure of the secondary and tertiary frequency.

barbaroja · Oct 18, 2019

Gold said:
From memory the three resonances in a Westrex IIID are: the primary resonance around 1K, the secondary resonance is about 13K and the tertiary resonance is around 20K. Take that with a grain of salt. I'm pretty sure there are three resonances but unsure of the secondary and tertiary frequency.

barbaroja · Oct 18, 2019

Some coils measured at Coil and XLR

barbaroja · Oct 18, 2019

Some more coils measured at Coil and XLR

barbaroja · Oct 18, 2019

Some coils measured at Coil and XLR

Gold · Oct 18, 2019

Looks like I was close.

barbaroja · Oct 18, 2019

What do you guys think about the differences between heads? 4 ohms should be a significant number of turns.

Any thoughts on this?

Gold · Oct 18, 2019

Heads #278 and # 289 look like what I would expect to see. The other three heads don’t look right to me.

barbaroja · Oct 18, 2019

Gold said:
Heads #278 and # 289 look like what I would expect to see. The other three heads don’t look right to me.

Weirdly enough, head 112 was the preferred one by some cutting experts that used them.

What do you think could be causing the differences in ohms. 112 is aluminum wound. Do not know about others yet. I would expect higher resistance in aluminum. Fewer turns somehow? That would mean less eficcient utilization of the magnetic flux if I recall correctly.

Beginner, simple circuit question

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