Line Output Converters

[SONARC]

Hi folks -

I was recently working on a couple of my old 1950s mono Newcomb record players, where my goal was to create a “Line Out” situation that would allow me to patch the Newcombs into modern stereo components. Part of the reason being that for a lot of older lo-fi records, they actually sound better on the Newcombs than than on my modern turntables (subjective, I know); and I’d like to digitally capture that pure vintage audio signal to store on SD cards, for listening on road trips and such.

The thing about the Newcombs is they do not have a Line Out or a headphone jack. The speakers for these units are old 2-prong Cinch-Jones types connectors that resemble small electrical plugs - one is positive, and one is negative, in tandem with the speakers themselves. Getting under the hood of the Newcomb unit allowed me to jump the wiring for the speaker jack, and run the signal into my stereo amplifier, so I could record a record on my computer as a waveform. Comparing that wave to another wave made from a modern turntable, I was able to more or less dial in the output level from the Newcomb, using the volume and tone controls. Not an optimal way to record, obviously, but it does work, and the signal was surprisingly clean.

From there, I started thinking about Line Output Converters as perhaps being a more efficient method of controlling the output signal from the Newcombs, and getting it more in line with the Line Out function of a modern stereo system.

Here are some of the key points I’ve gleaned from looking at various Line Output Converters:

• connecting factory speaker wire from the stereo to an aftermarket amplifier
• converting 2 channels of speaker wire to RCA outputs
• adding an amplifier to an OEM system

Okay, now moving on to why I’m talking about this in the Lathe Trolls Forums....

When I first attended a Lathe Camp in Fall 2016, one of the first things that jumped out at me was that there was something like 8 stages of gain between the sound source and the cutting head. To me, that suggested introducing a lot of line noise, whether audible or not, into the signal.

Currently, my signal chain consists of running the RIAA audio from the Line Out on a Fiio player > EQ > cutting head. I suppose that technically, I *could* make any tweaks to the audio with the EQ and then create a new track from that, and then the chain would be: Fiio > cutting head; but that seems like a lot of unnecessary back and forth conversion for what might possibly be just an occasional nudge on the EQ.

But this Newcomb thing I’m working on got me wondering if anyone here who works with the Presto, Rek-O-Kut, Wilcox-Gay, units etc, might have explored the possibility of using some sort of Line Output Converter to regulate the audio signal going from whatever the audio source is, going into the cutting head? It seems like that would work pretty much the same as using a cassette deck with your stereo amp, where you’re only adjusting the input on the deck, and everything else is a fixed level signal.

Trust me, I have very little electronics knowledge, and unfortunately, despite my best efforts over the years, I’ve never really been able to get my head around a lot of the theory that a lot of folks discuss in these forums. So if this seems like a crazy idea to anyone, it would be helpful if you could tell me why, in layman’s terms that I might be able to understand. But if anyone has actually used something like this in their own process, I’d love to hear about it - what factors come into play, since there seem to be many different types of converters out there, and what to consider if trying something like this.

Thanks in advance for any info anyone might be able to offer with this!

[tragwag]

what I gather from your post is - why not simplify the signal chain as much as possible?
I respect that other cutters may have a different way of working, but for my Presto 6N setup it goes like this:
focusrite sapphire pro 40 ->mackie big knob (routing and level only) -> power amp ->cutting head

I do my I-RIAA, head corrective EQ, and taste EQ in the box, along with some de-essing and usually compression or limiting depending on the source.
With the way the Big Knob is routed, I can monitor in stereo and mono with the same source.

I am curious if the line output you mention might function like a buffer amp, but that would only be necessary if you brought the signal down significantly in the analog domain before the power amp

[SONARC]

Hi trag - Sorry, I didn't realize until recently that there had been any replies to my question - I forgot to switch my notifications on!

Yes, to some extent, I have been determined to try and limit the number of gain stages the signal needs to travel through, which is why I go straight from the Fiio (Line Out) > EQ > power amp > cutting head. It sounds similar to what you are running. However, I'd still like to be able to strip it down to something like: source > cutting head. That would mean establishing *everything* (RIAA, EQ, Amp level) for the source audio, and creating the simplest chain possible, bypassing all other knobs, faders etc. Admittedly, maybe that kind of talk is completely crazy, but there ya go. Anyway, that's why I was thinking about the line converters. That said, I’m not sure exactly what is meant by a “buffer amp” - I mean, the way you describe it, I sort of get it, but I’m trying to see if it’s possible to eliminate the power amp from the chain as well.

For my Newcomb digitizing tests, I guess technically what I was doing was applying a primitive method of converting Speaker Level to Line Level. I'm absolutely not an electrician, but it seemed to me that the *reverse* concept could possibly be applied to lathe-cutting, since the circuitry of a cutting head is more or less Speaker Level, and the source audio could be considered Line Level. I don't know if the way I'm describing any of this makes sense to the folks here who are way more versed in electrical theory than I am, but if I've ever learned anything, it's that for every concept or theory one can ever imagine, someone else has already done it.

what I gather from your post is - why not simplify the signal chain as much as possible?
[ - snip - ]
I am curious if the line output you mention might function like a buffer amp, but that would only be necessary if you brought the signal down significantly in the analog domain before the power amp

[tragwag]

interesting! I'm not sure I'm fully understanding how one could eliminate the power amp in this scenario, except by using an integrated amp.
an occasion to use a buffer amp would be in your signal chain after doing subtractive EQ (if the EQ itself doesn't have makeup gain)

[dmills]

You can eliminate a lot of the gain stages in eq and limiting by going digital for that, but the power amps sort of need to be actual amplifiers, and anyone running a feedback head needs to close that loop in the analog domain simply because the group delay thru a set of ADCs and DACs will destroy any chance of loop stability at reasonable feedback levels.

You would want to structure things such that a full scale glitch at the output of the digital doings does not cook the cutter....

I would note that a gain stage done with relatively modern doings and designed to minimise Johnson noise is usually so quiet as to be a complete non issue in line level doings, anyone designing electronics doesn't think twice about them if you can keep the impedance WAY down, because the humble resistor is usually the dominant noise source, not the opamp.