A Modern Loftech TS-1 Audio Test Set

Where we discuss new analog design ideas for Pro Audio and modern spins on vintage ones.
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mediatechnology
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Re: Loftech TS-1 Schematics

Post by mediatechnology » Fri Nov 24, 2017 2:59 pm

JR. wrote:
Fri Nov 24, 2017 1:08 pm
I am not enthusiastic about trying to merchandise this, but tempted to make one, because I can... 8-)

JR
And you should.

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JR.
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Re: Loftech TS-1 Schematics

Post by JR. » Sun Nov 26, 2017 11:50 am

I found a nice back lit LCD display with 16, 1/2" high characters... not cheap (>$20) but with 16 characters I can display decibel level and frequency at same time.
Image

There are cheaper displays but with much smaller characters. The only cheap/large character LCD was 4 digit clock display....

JR

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Re: Loftech TS-1 Schematics

Post by JR. » Sun Nov 26, 2017 2:52 pm

This is fun thinking about this....

I could use a 16B DSPic that has 2x16b DAC built in, but only 12 bit A/D built in... Or in the $3-$4 range I can get a 24b stereo in/stereo out codec... that I can hang on a cheaper micro...

Probably use a DPOT to pad scale the input range down to the 3V codec.

Now that is probably more bits and I/O than I need, but why scrimp?

I need to soak this in beer tonight. 8-)

JR

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mediatechnology
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Re: Loftech TS-1 Schematics

Post by mediatechnology » Sun Nov 26, 2017 5:06 pm

After soaking it in beer you might want to start a thread in the Design area of the forum. I'm liking this.

Other than the analog front end for the measurement A/D and the balanced output there doesn't seem like much additional outboard circuitry would be required.

Rather than a DPOT you might consider a resistive pad since the DPOTs THD would limit you if you ever wanted to use the same engine to perform THD measurement. You would want a low distortion coarse output pad too wouldn't you?

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Re: Loftech TS-1 Schematics

Post by JR. » Mon Nov 27, 2017 11:27 am

Wayne can you split this topic and move this veer to another forum?

Moderators should have that capability.

If not I will start another.

JR

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mediatechnology
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A Modern Loftech TS-1 Audio Test Set

Post by mediatechnology » Mon Nov 27, 2017 12:00 pm

I will move that. Not something I do often so I have to remember.

What shall we call it?

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Re: Loftech TS-1 Schematics

Post by JR. » Mon Nov 27, 2017 1:10 pm

mediatechnology wrote:
Sun Nov 26, 2017 5:06 pm
After soaking it in beer you might want to start a thread in the Design area of the forum. I'm liking this.

Other than the analog front end for the measurement A/D and the balanced output there doesn't seem like much additional outboard circuitry would be required.

Rather than a DPOT you might consider a resistive pad since the DPOTs THD would limit you if you ever wanted to use the same engine to perform THD measurement. You would want a low distortion coarse output pad too wouldn't you?
Distortion? AFAIK these are pretty clean... (I built a prototype automatic mixer using DPOTs for gain control and they sounded good to me) they use real resistors and modern TG technology to switch the digital wiper. This is the same technology as used to switch gain inside digitally controlled mic preamps and they deliver respectable THD. While even modest distortion would not affect simple audio level measurements.

From a quick glance the cheap 24b codecs only deliver a -108dB analog noise floor, but I can work with that (better than old TS-1). I had early prototype TS-1s measuring -100dBu because the limited gain bandwidth of simple rectifiers, reduced bandpass so much the noise floor looked better than it was.... I might have sold more units if I didn't correct that. :roll:

It might be nice to add some weighting noise curves, but I need to be careful about mission creep. I might be able to do simple filters w/o full dsp instruction set.

Making this battery operated would limit output level but really simply noise floor management (and design). Portable battery operation would be a really nice feature...

The old one used a section of TL074 for output so drive capability was already pretty limited. Perhaps a step up audio transformer could match +20dBu output level (while might add some LF distortion) into modest impedances (not 600 ohm).

I need to resist making this stereo just because stereo codecs are cheap... I could use the two inputs in differential mode (or not).

Two outputs do not necessarily add any compelling new features, so nah. Better yet I need to find a cheap mono codec but most of them are oriented toward voice bandwidth. Some with single voice input and hifi stereo output but that is opposite of what I need.

need to KISS

JR

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Re: A Modern Loftech TS-1 Audio Test Set

Post by mediatechnology » Mon Nov 27, 2017 1:32 pm

I think a single generator output would be fine.
The ability to have two inputs would be nice though for phase, crosstalk measurements or generator monitor.

Do you suppose a switcher for the output stage to produce rails for +21/27 dBu would be possible?

If the DPOTs are as good as the attenuators in the THAT gain controllers then they're good enough. I had a hard time measuring their THD when used as attenuators.

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Re: A Modern Loftech TS-1 Audio Test Set

Post by JR. » Mon Nov 27, 2017 2:13 pm

mediatechnology wrote:
Mon Nov 27, 2017 1:32 pm
I think a single generator output would be fine.
The ability to have two inputs would be nice though for phase, crosstalk measurements or generator monitor.
I like that... done
Do you suppose a switcher for the output stage to produce rails for +21/27 dBu would be possible?
Possible, and it would only need to run when elevated output levels are requested so not kill the batteries. I like that better than adding an audio output transformer to step up level. May even consider a class D output stage to drive 600 ohms (or not).
If the DPOTs are as good as the attenuators in the THAT gain controllers then they're good enough. I had a hard time measuring their THD when used as attenuators.
Yes, I planned to use them instead of VCAs Only issue for modulating audio audio gain is switching glitches and the modern parts I messed with were pretty good even for that... In my prototype I planned to coordinate gain changes with zero crossings and never needed to implement it up to the stage of development I reached (but never finished so critical listening may have forced me to finish the zero crossing logic).

JR

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Re: A Modern Loftech TS-1 Audio Test Set

Post by mediatechnology » Mon Nov 27, 2017 2:25 pm

If the codec you end up finding is only available in 2 in/2 out maybe you could use the second D/A to provide the generator anti-polarity output or in parallel to improve the generator noise by 3 dB.

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