Battery power for USB interface

F

Findlay

Member
This is probably going to sound like a really weird post - and I apologise in advance as it sort of falls between analogue and digital and I'm not sure where to post it!

I'd like to be able to use my Tascam US122 mkII USB interface sometimes on battery power and not plugged into the pc. This is because I would like to use the excellent mic pre-amps in this to feed a line level output to a pocketstudio. The pocketstudio is fine with condenser mics but awful with my Shure Unidyne III dynamic - yet the 122 sounds wonderful with this old mic and I'd like to use it if possible. I want to avoid having to plug it into the pc USB socket because it makes everything too unwieldy and the noise from the laptop fan is intrusive - one of the things I'm trying to get away from, together with using a pc for recording. I've tried using a USB battery pack but the green USB power light on the 122 doesn't light up and the inputs don't work - oddly the red MIDI light comes on. I know this all sounds a bit strange but does anyone know if there is a work around for this? Grateful for any help!
 
The drivers that are in the PC as well as some other stuff are what tells the firmware in the US122 how to function. You need the PC connected for the US122 to have it work. I went through a similar 'experiment' trying to get a US144kmii to work without the PC.

The old Unidyne mic needs more gain than your condenser mic and perhaps pushing the gain on the US122 brings in some noise. A possible work around might be something like an inline preamp like a Fethead which is powered by phantom power and gives a dynamic mic about 25dB of clean gain.

https://www.bhphotovideo.com/c/prod..._audio_fethead_in_line_microphone_preamp.html
 
arcaxis said:
The drivers that are in the PC as well as some other stuff are what tells the firmware in the US122 how to function. You need the PC connected for the US122 to have it work. I went through a similar 'experiment' trying to get a US144kmii to work without the PC.

The old Unidyne mic needs more gain than your condenser mic and perhaps pushing the gain on the US122 brings in some noise. A possible work around might be something like an inline preamp like a Fethead which is powered by phantom power and gives a dynamic mic about 25dB of clean gain.

https://www.bhphotovideo.com/c/prod..._audio_fethead_in_line_microphone_preamp.html
Click to expand...

Thanks for this.. Good to know that you tried it too! That amp looks a great bit of kit but the price doubles on import to the UK. The level from the mic is on the low side but not bad. I am wondering if it is an impedance issue with the dp08 inputs and that a transformer may help?
 
The impedance of a DP008 (you had said DP08(?)) is 2.4kohms which is OK for the mic and shouldn't need a transformer.

You could give a budget preamp like this .... https://www.thomann.de/gb/art_tube_mp.htm ..... a try and see if it helps, but not knowing what you mean by 'sounds awful' it's hard to know what the problem might be. The ART preamp is not an expensive pre and won't sound high end, but it may give the Unidyne a boost that may help.

From what I can find in the Unidyne III it appears it can set for low or high impedance by a 'socket' selector in the connectors base per the PDF manual I found. I would assume it's set for low impedance if it works OK in the US122.
 
arcaxis said:
The impedance of a DP008 (you had said DP08(?)) is 2.4kohms which is OK for the mic and shouldn't need a transformer.

You could give a budget preamp like this .... https://www.thomann.de/gb/art_tube_mp.htm ..... a try and see if it helps, but not knowing what you mean by 'sounds awful' it's hard to know what the problem might be. The ART preamp is not an expensive pre and won't sound high end, but it may give the Unidyne a boost that may help.

From what I can find in the Unidyne III it appears it can set for low or high impedance by a 'socket' selector in the connectors base per the PDF manual I found. I would assume it's set for low impedance if it works OK in the US122.
Click to expand...

Thanks guys. Sorry about the DP008 typo - I was a bit tired last night! I have the same problem with a DP004 and a DP01FX - with any dynamic mic plugged into these there is a lot of hiss - about the same level as you get with a cassette deck running type I tape with no NR. The tonality is fine. They all specify around the same input impedance but when I use my ancient Tascam 244, which has a spec input Z of 10kOhms there is no hiss and the mic sounds great. I have been using this as a pre-amp a few times but it is a bit daft alongside the pocketstudio! I'll take a look at the preamp you suggested. I'm a bit loath to change the impedance at the socket because the socket is a bit weird and the wiring is a bit fragile in this old mic! I'll look at Symetrix too Garww.
 
There are USB interfaces that will run "Stand Alone" and those that won't. Even those that will often give a limited number of functions.

My old M-Audio Fast track pro would run SA and indeed, my son uses it as a local mic pre to feed a PA over a long cable since it gives him a local VC. He could also use the high Z input on the other channel if he liked. S/PDIF I/O is also active but you have to setup these function with a PC beforehand.

However, many of the newer AIs have dropped the facilities of earlier models (most of you know of my MIDI rants!) so I would not be at all surprised that the 122 does not give SA capability. There is one possible ray of light? Have you measured the supply you are using for the Tassy? It might be that it does not quite get to 5V and therefore some sections will not turn on. My F'rite 8i6 is fussy about the 12V you give it, needs at least a 1.5A supply, one amp won't cut it. (not that the 8i6 will run SA AFAIK.)

Dave.
 
"They all specify around the same input impedance but when I use my ancient Tascam 244, which has a spec input Z of 10kOhms there is no hiss and the mic sounds great."
What kind of plug has the Unidyne mic got on the end of the cable, XLR or 1/4"TS? I've got an old 246 which only has 1/4" mic/line ins and as well the 244 is the same it appears. If the mic cable does terminate to a 1/4" plug there's a good chance the mic may be set to high impedance. I did need to use a transformer with the old 246 to terminate low-z XLR mics. Perhaps you're using a transformer with the 244(?)
 
Findlay said:
Thanks for this ray of hope!

Only guessing but...The M-A FTP was USB 1.1 and that was slooooooow so it might have been that they put all the processing inside the unit (actually must have done because it was a stand alone S/PDIF converter both ways) ? USB 2.0 is fast enough that 2 way communication is possible with the PC but very few mfctrs used really good drivers.

I do recall that despite being 1.1, the pro had very low latency when MIDI was triggered by a dumb kbd. In fact I always suspected that the pro was just a 2496 card in a tin with mic pres!

BTW, I have had both a 122 and a 144 and both had pretty shitty low gain, fairly noisy mic pres? To be fair, the pro's pres were pretty poor, that's why I bought some capacitor mics, an LDC and 2 SDCs.

Dave.
Click to expand...
 
The Unidyne has an XLR plug at the end of the cable. I use an XLR to 1.4"TS cable for the 244. Using a DVM it measures up as 25 Ohms - not like measuring impedance I know, but still low. I bought a Chord IM07 the other day which measures up at 600 Ohms and behaves exactly the same way connected up using XLR to the DP008 (noisy) or using 1/4"TS to the 244 (clean, great sound, no noise). I'm not using a transformer with the 244 - I'm wondering if this would help with the DP008?
 
I think I've finally found a partial solution to this - not a battery powered USB interface but a cheap battery powered pre-amp to improve the inputs of the Tascam digital recorders when using dynamic mics. I've been playing around with an old NE5332 op amp I had in the drawer and with just a couple of resistors and a couple of 9V batteries it is producing about 50dB of gain. Plugging in the Shure or the Chord there is virtually no noise and pretty decent frequency response when the output is fed into the Tascam DP004 line in. I'm using the inverted input of the op amp with a very low value input resistor to give low impedance. I was expecting a fair bit of distortion with a single high gain stage but I can't hear any. It is miles better than using the DP004 mic in! I think these recorders (and maybe lots of others) may use a non-inverted op amp input and that the high impedance doesn't suit these mics. Am I completely wrong here?
 
Findlay, you are on the right track!

I would suggest the use of a 1:3 or 1:4 input transformer and OEP Electronics makes some very good ones at fair prices. Run that into a 5532 if you like but the single NE5534 is even better for low noise, about 5dB. Even better still is the LM4562. A dual op amp it has even lower noise than the 34 and super drive capability (virtually no distortion even into 600Ohms) .

The combination of a traff and an NE forms the basis of the Focusrite 'Red' pre amp so you are in very good company.

One day (ha!) if I can get my lazy arse into gear, I am going to build a pair of pres' with the 4562 +traffs as very low noise preamps for the dynamics in my garden recording wildlife. Headroom does not bother me as the only thing that will 'crack' them are cars. a local piston engined 'tripping' plane and the occasional Sonic Boom!

Oh! Also, input traffs make things very RFI proof.

Dave.
 
Thanks for this Dave. What effect does the input transformer have?

Asking a possibly silly question, I'm wondering whether to build another amp with a balanced input. At the moment I'm using the mics unbalanced, with the "cold" lead wired to the earth on the jack. I realise you lose the noise-cancelling effect of running balanced but wonder if you also lose signal this way? The problem is, as I see it it is a bit more complex building a low Z pre amp with a balanced input. I'm thinking I would have to have two op amps (or dual) as the first stage both wired for inverted input, then run the "hot" output into another inverted op amp to give a positive signal then sum both signals in another stage. Am I wrong here or is this what you have to do, and is it worth all the bother? I seem to get pretty good results as it is!
 
Well, as they say, "If it ain't broke..."? Yes, you COULD build a balanced input amp using a single stage of an NE5532. The two input resistors would need to be about 560Ohms so as to present a load of about 1k* to the mic and a feedback R of around 16k would give you 30dB or so of gain then you could configure the second IC stage as a variable gain 'make up' amplifier. But such a stage would not have very good 'common mode rejection' i.e. it actually would not be a good balanced input and it would not be optimized for noise. Again you are right, a proper job would be a pair of op amps to buffer the input feeding a low Z balanced stage. In fact peeps don't bother they use a pair of ultra low noise discrete transistors in the front end and that beats almost all ICs for noise.

Transformer: First off it balances the input. The transformer will only pass a signal that is alternating + and - at each end of the primary windings any signal common to each winding is ignored. Because traffs 'resist' such signals techs often speak of a 'common mode impedance' and this can be huge for a transformer, 50 meg Ohms, no standard solid state input can get close to that.

Next goodie is free gain! I suggested a 1:4 ? That is a step up ratio of 12dB and that level boost is noise free and means the following electronics need have 12dB LESS gain than without a traff and that means 12dB less hiss!

Transformers are seriously lossy past a few tens of kHz so RF does not easily get past them. This RF rejection can be greatly improved by an 'interwinding shield' a sheet of foil twixt pri and sec' that is earthed.

People also wax lyrical about 'transformer sound' but mic input transformers at least can rarely be driven hard enough to show much 'attitude'.

Dave.
 
* Actually it won't quite be the 1120 R you expect because the non inverting resistor gets 'anti-bootstrapped' and appears lower.

No! I don't quite understand it either but Duggy Self says it is so and I believe him!

Dave.
 
Thanks again Dave for all this. It is so helpful! I think I will just leave well alone! One thing which is bugging me is that I am using almost zero ohms at the input to the op amp. I originally put a 50k resistor in the feedback loop to the inverting input and a 1k trimmer at the input but found I had to set the trimmer to zero to get a good output. The gain seemed to zap up very fast to about 50 dB at this point. It seems weird but it sounds wonderful! Any ideas?
 
Findlay said:
Thanks again Dave for all this. It is so helpful! I think I will just leave well alone! One thing which is bugging me is that I am using almost zero ohms at the input to the op amp. I originally put a 50k resistor in the feedback loop to the inverting input and a 1k trimmer at the input but found I had to set the trimmer to zero to get a good output. The gain seemed to zap up very fast to about 50 dB at this point. It seems weird but it sounds wonderful! Any ideas?
Click to expand...

Aha! You have wire the NE as an 'inverting' amplifier. Gain is therefore R feedback (50k) divided by R or Z in. The mic is probably around 300 Ohms (despite what the speccs say!) so that gives you a gain of around 45dB even WITHOUT the series 1k.

You could make the feedback R a 100k pot with say 10k in series as a 'min gain stopper' That gives a gain range of approx. 50 to 30dB but you can play with those values. Have you got the mic DC coupled to the chip input? If so, naughty. There should be a DC blocking capacitor, 47mfd at whatever supply rail voltage you are using.

A better design is to configure as a 'non inverting' stage when the feedback and gain control is separate from the mic input circuitry, still need a coupling cap' but also a bias resistor to 1/2 supply. This could be around 2k2 and thus a common load value for mic pres.

If you want to persevere with mic pre construction a good practice is to use an 8 pin DIL socket for the chip. NE5532 are pretty cheap and indestructible but TL072s are even cheaper and just as tough. You can then play silly buggers with the TL0 and once things are working, plug in the NE. The TL072 will do everything the NE will but will be noisier and not drive loads below 2k or so very well.

Dave.
 
Should have said...If you do fit an input cap to the present circuit you will then need a bias resistor to 1/2 supply. Anything handy will do, 10k or so.

Dave.
 
Thanks again. I don't have a de-coupling capacitor at the chip input - I thought I could get away without it because I'm using a dual rail supply (two batteries). I'm a bit wary of the loss of bass response, but I guess 47mfd allow for good bass? I guess I went for the non inverting design because it seems to match the low mic impedance beautifully - I'll try the non inverting one out and use your idea of experimenting with the cheaper chips. I've ordered an LM4562 for the next design.

I've got the output from the present inverting chip going to a 10k resistor and a 10mfd capacitor. Do I really need these?
 
You must log in or register to reply here.
Back
Top