Transfer Filter / Neumann HT75 Replacement

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mediatechnology
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Transfer Filter / Neumann HT75 Replacement

Post by mediatechnology » Tue Feb 09, 2021 7:35 am

This project is a 18 dB per octave high and low pass filter to replace the Neumann HT75 that is often used for vinyl cutting.

The HT75's most useful frequencies of 40 Hz and 16 kHz were chosen for general-purpose band limiting.
The filters are 18 dB/octave Butterworth alignment with each section having relay bypass.
Balanced inputs and outputs are on-board but can be removed and linked to provide unbalanced connections or to cascade filter boards.

Image
Transfer Filter TF1 to replace Neumann HT75.

Image
Transfer Filter TF1 to replace Neumann HT75.

For someone wanting additional frequencies multiple boards can be cascaded.
Two boards can also be controlled in parallel for Modulation plus Preview.

The relay supply and ground connections are isolated and floating from audio ground.
An on-board regulator can be used with 24V relays to operate the filter from the +/15V (30V) audio supplies.

An additional application for the TF1 is a playback warp filter.
Component value changes in the HP section convert it to an 18 dB/octave 18 Hz filter.

The initial run of boards have been sold.
Watch this space for availability of the next run.
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Re: Transfer Filter / Neumann HT75 Replacement

Post by Gold » Tue Feb 09, 2021 3:05 pm

Thanks Wayne! I am the one with the limping HT75's. Hopefully I'll be able to get them built and installed in the next week or so. I can't wait decommission the HT75's for a few reasons. I will be able to locate the PCB's in the same chassis as the EE. This will free up 2RU for future expansion.

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Re: Transfer Filter / Neumann HT75 Replacement

Post by JR. » Wed Feb 10, 2021 10:00 am

Not remotely comparable but back in the 70s/80s my kit business sold a very inexpensive kit (P-91?) that provided 2 pole HPF and 2 pole LPF, I'm a little sketchy on the details but as I recall I shipped it with a single set of capacitors, but multiple resistor sets so my customers could select different cut off half power frequency points.

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Re: Transfer Filter / Neumann HT75 Replacement

Post by Gold » Wed Feb 17, 2021 3:59 pm

Because of the weather my Mouser shipment was delayed. I've done a bunch of work on the boards so it hopefully won't be too much work to get them installed . I've stuffed and soldered one board to make sure it works. I've stuffed and soldered most of the other boards except for the tuning capacitors and resistors. I'm holding off on those until the first one works. I have them mounted on the PVC rails I've been using. I opened up the EE chassis yesterday and it looks like these will drop in without too much trouble.

Image

Image

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Re: Transfer Filter / Neumann HT75 Replacement

Post by mediatechnology » Thu Feb 18, 2021 9:24 am

Thanks for posting the pics Paul.

I like your method of recording the values on the tape.
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Re: Transfer Filter / Neumann HT75 Replacement

Post by Gold » Thu Feb 25, 2021 3:32 pm

I have all four boards working and tested. Hopefully I can finish up the unit on Saturday. Since it's going in the case with the EE's the wiring shouldn't be too bad. I can recycle some of the wiring already in place.

Image

Image

I still don't have my FFT rig together to the point where I'm comfortable with it. To test the filters I measured gain an octave away from the filters corner frequency. For the HPF I measured at 20Hz. For the LPF I measured at 32K Hz. I hand matched the components as closely as I could. Mostly to better than 0.1% but not on everything.

Mod A
HPF Left: -13.58dB
HPF Right: -13.53dB

LPF Left: -13.94dB
LPF Right: -14.00dB

Mod B
HPF Left: -13.86dB
HPF Right: -13.98dB

LPF Left: -13.73dB
LPF Right: -13.75dB

Pre A
HPF Left: -13.63dB
HPF Right: -13.76dB

LPF Left: -13.99dB
LPF Right: -13.98dB

Pre B
HPF Left: -14.00dB
HPF Right: -13.04dB

LPF Left: -14.02
LPF Right: -13.95

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Re: Transfer Filter / Neumann HT75 Replacement

Post by mediatechnology » Thu Feb 25, 2021 4:03 pm

That's excellent Paul!

The only one that was significantly different was:

Pre B
HPF Left: -14.00dB
HPF Right: -13.04dB

I did a response error sweep on mine - the difference we see in the pass band is mostly gain error in the measurement A/D.

Image
Transfer Filter Sweep HP and LP In Magnified

Image
Transfer Filter Sweep HP and LP In
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Re: Transfer Filter / Neumann HT75 Replacement

Post by Gold » Thu Feb 25, 2021 4:16 pm

mediatechnology wrote:
Thu Feb 25, 2021 4:03 pm
That's excellent Paul!

The only one that was significantly different was:

Pre B
HPF Left: -14.00dB
HPF Right: -13.04dB
Yeah, I ran out of very tightly matched sets. I was selecting for absolute value as well as relative matching. That's why the gains also match from board to board. I could get a better left right match on Pre B but the overall gain would be more off from the other boards. I might give it another shot. I have have the measured values written on the cap so I could probably improve it. I'll put the filters before the EE so any HPF gain difference won't affect the automation with the EE in.

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Re: Transfer Filter / Neumann HT75 Replacement

Post by Gold » Thu Feb 25, 2021 7:59 pm

It looks like the error in the Pre B HPF is an errant resistor. I went back and found better capacitor matches. It helped a little but not much. I'll have to go through the resistors again.

I have a couple of questions though. I tried to use exact specified values for the capacitors and resistors. I get a -3dB point closer to 17K. It is -18dB at 37.5k. Is that within expected tolerance?

The HPF is better. The -3db point is 39Hz and the -18dB point is 18Hz. Both measurements done with the Mod A board.

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Re: Transfer Filter / Neumann HT75 Replacement

Post by mediatechnology » Thu Feb 25, 2021 8:37 pm

Gold wrote:
Thu Feb 25, 2021 7:59 pm
I have a couple of questions though. I tried to use exact specified values for the capacitors and resistors. I get a -3dB point closer to 17K. It is -18dB at 37.5k. Is that within expected tolerance?

The HPF is better. The -3db point is 39Hz and the -18dB point is 18Hz. Both measurements done with the Mod A board.
Yes, that's about as close to 16 kHz as we can get on the LPF with E6-series capacitors.
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