THAT1240 Gain Tricks

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mediatechnology
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THAT1240 Gain Tricks

Post by mediatechnology » Thu Feb 01, 2018 2:57 pm

Gold wrote:
Thu Feb 01, 2018 12:51 pm
mediatechnology wrote:
Wed Jan 31, 2018 4:38 pm
By switching the THAT1240 in a unique way its gain can be varied from -6, to 0 and to +6 dB. This eliminates the requirement for additional gain or attenuation stages that would add a differential phase delay to one process path but not the other. High frequency crosstalk performance is maintained.
This looks great Wayne. It's much more product like. Could you expand on the gain switching of the 1240's? I don't think I've seen that before.
Image

Image

The -6dB summation was a real "doh!" Homer Simpson moment. So obvious.
The identical value input resistors, internal to the 1240, form a 1/2 voltage divider for the non-inverting input and reference input when driven by 0Ω sources.
By connecting the inverting input and sense input of the 1240 back to the output to make it a follower, the 1/2 attenuation of the passive divider is preserved.
On the current MS Matrix pin 2 is grounded and the mono sum for identical inputs is +6dB.

The bias resistors that are shown in three of the configurations are required if the input floats.
(The bias current is typically around 900 nA IIRC.)
If they're connected to op amp outputs the resistors are somewhat redundant.
I put them in to aid troubleshooting in case the driving stage is removed from its socket.
When a relay is switching the input bias resistors are required.
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Re: THAT1240 Gain Tricks

Post by emrr » Sat Feb 03, 2018 2:10 pm

good one
Best,

Doug Williams
Electromagnetic Radiation Recorders

billshurv
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Re: THAT1240 Gain Tricks

Post by billshurv » Fri Feb 09, 2018 7:13 pm

Hi,

Delurking here for the first time. Wayne persuaded me to join when he realised how bad he potentially was for my wallet :).

Currently working on the plans for a new pre-amp to handle the pure hedonism of having two turntables. As part of the inevitable early phase mission creep I've been considering the options for mono. Now there are many ways to do mono and I want to be able to support the following.

1. Parallel strapping of the cartridge feeding both inputs
2. Series strapping of the cartridge feeding left input
3. standard Electrical L+R summing.


On first pass these all look simple until I realise that the MM stage I am using has a rather odd feature. It's a transimpedance stage and, to a first order auto adjusts it's gain. So a series strapping doesn't give you a left channel 6dB hotter than stereo. I could just accept I have to crank the gain 3 notches in that case, but it would be nice to keep the level the same for each possibility. This gives me a couple of easy options for this.

The most annoying part about this is that if only miniDSP had more presets I could do all this bit digitally.


And before anyone points out, I know that the different mono methods should in theory produce exactly the same output, but there are some possible benefits to a series strap I want to test. I also have a possible plan to get one of my cartridges convered to produce M+S natively (rotate coil 45 degrees).

Bill

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mediatechnology
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Re: THAT1240 Gain Tricks

Post by mediatechnology » Fri Feb 09, 2018 7:28 pm

Thanks for joining us Bill.

Mono being lateral modulation? Early LPs?

My instincts tell me series strap.
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Re: THAT1240 Gain Tricks

Post by billshurv » Sat Feb 10, 2018 12:18 pm

Yeah monos. I've started collecting mono Decca recordings. There are various arguments about the best was to do these things but would be nice to have a way to test them.

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Re: THAT1240 Gain Tricks

Post by mediatechnology » Sat Feb 10, 2018 3:34 pm

I think I have one client that uses parallel with a 23K5 load.
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Re: THAT1240 Gain Tricks

Post by JR. » Sat Feb 10, 2018 4:33 pm

I probably have some old mono vinyl LPs that haven't been touched for over half a century, and won't be by me.

Nothing of value I'm sure, my dad was a recording engineer and he had some decent classical/jazz open reel tapes (also mono).

What is your taste in music, I ASSume audubon bird calls are not in high demand.

There were some old risque (blue) comedy records.

JR
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Re: THAT1240 Gain Tricks

Post by billshurv » Sat Feb 10, 2018 5:52 pm

I Actually do have a couple of records of birdsong. Has done me no good in being able to recognize birds from their calls though. I don't know if it's some fake nostalgia, but the best recordings of the mono era do stand up very well. OK nothing over 15kHz but the simple recording techniques and basically live performances can have a lot of power. Choral music is less successful and large orchestras often distort. But as I get older I'm in it more for the performance.

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