HELP! Struggling to find the right amp!

spamjam

Member
2010-11-12 3:54 pm
Hi,

I am new to the forum so please forgive me if I have posted in the wrong place or something!

I have a slightly unusual problem in that I am trying to find an amplifier that can generate a voltage of up to about 70V as part of a scientific project I am working on. Previously I have seen the Rolls RA2100b and the Crown DC300A used for this but I cannot obtain either of these and don't know what I should use as an alternative. I am new to this field and so am not familiar (yet) with all the terms and details of the amplifiers, although I am learning!

The signal I need to amplify is a square pulse and so to avoid distorting the signal it is my understanding that the THD and the frequency response need to be as good as possible?

My query is whether a 'standard' audio amp (such as those available from hi-fi stores) can generate this kind of voltage as these seem to offer the best in terms of distortion and frequency response? I have been trawling through the manufacturers details but am not seeing anything that relates to voltage output, just the power ratings. Is there a way of working out the voltage from this?

As the budget is is quite tight I cannot afford to spend thousands of pounds on this, so far I have had a range of PA type power amps suggested but I am unsure if this would do the job?

In the long-term I would like to build something for the job but at the moment I need to get hold of one asap. Any help/explanations/advice would be very much welcome!

Thanks! :)
 

bobodioulasso

Member
2008-05-02 12:06 am
You speak about voltage only. Though, we need to know more about the load you intend to use. Do you need power or voltage only?
Is it resistive, reactive? How much current do you need into that load.

An audio power amp is intended to produce power, not voltage only.
 

spamjam

Member
2010-11-12 3:54 pm
Hi,

Thanks for your reply, I will try to include more detail but as I am new to this field there are many things I do not know! :confused:

The voltage signal produced will be amplified by the amp and then passed on to wire electrodes and then through artificial tissue that would probably be high resistance. My understanding is that it is the voltage that we are after and that the power is less of an issue. the main thing is to produce a stimulation voltage of something in the range of 20-70V with minimum distortion of the square pulse.

I hope that this helps in some way, it is quite hard to explain what I am trying to set up - please forgive my ignorance on this topic at the moment, trying to learn!
 

spamjam

Member
2010-11-12 3:54 pm
OK thanks, if an audio amplifier would maybe not be the best choice then can you suggest any other options that I could look into? I have mainly been looking at audio/PA places and RDL too but haven't really got a clue where to look for alternatives.

I would love to be able to build one but as I am so inexperienced and just trying to get my head around using LabView software at the moment too to build this system it would be easier to buy one at this stage and build one if necessary later when I know a bit more about it!

:)
 

spamjam

Member
2010-11-12 3:54 pm
Thanks Nico Ras - I think the pulse width will vary depending on the experiment - ranging from 0.2-2 ms for example. The pulses will either be a single pulse, or a train of pulses at varying frequencies in the range of 20-100Hz.

What is a TEMS device?
 

bobodioulasso

Member
2008-05-02 12:06 am
At this frequency, an audio amp will do the job.
Though it would be a waste as you do not need power.
You need a voltage amplifier. Or an audio amp without his output stage. And without the need of a big powerfull transformer.
If it was me, i would use an LME49830 powered at +/- 75 volts with a small (20Va) transformer, and without output stage.
You can google for lme49830, download the pdf, and look at p 6.
This circuit diagram shoud meet your requirements if you do not need more than 50mA.
Just an idea. Any comment?
 

spamjam

Member
2010-11-12 3:54 pm
Hi Xoc1 - I am not sure but I think the amp I have seen used for this was used in 70V bridged mode. However I was working on the understanding that I was after 70V relative to ground - but I may be wrong, what is meant by having both terminals floating?

smartx - Sorry not really related to that, interesting idea though! :)

Thanks for the responses guys, really helpful and I'm learning a lot about this! :)
 
A machine like this is to stimulate muscles, The "ground" electrode is placed on a motor point, say the lower back while the floating electrode is moved over the muscle that needs stimulating. Stimulation of paraplegics muscles can require as much as 170V impulses. The absolute squareness is not that important but there should be no spikes or overshoot as this have very different effects. - How am I doing so far?

Different stimuli may need a train of pulses while other applications need single pulses or trains of pulses with varying mark space rations etc.
 
Typical TEMS

Here is a typical TEMS machine.
 

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spamjam

Member
2010-11-12 3:54 pm
Hi bobodioulasso - yes that was my understanding, but I may be wrong! :) Would this mean that I couldn't use a bridged amplifier as Xoc1 says? I have found a power amp at home that I may try just to see what kind of signal I can get and then go from there. In the long run I would love to build something myself though and might start to get together some bits to have a play around and see what I can produce!

Hi Nico Ras - Wow that looks really interesting - the Winks Greene Transeva can go up to 390V according to Google, not sure I'd like that on my muscles! Actually the project I am working on is to stimulate muscle but NOT real muscles attached to a person or anything - but to all intents and purposes similar in terms of resistance etc I would imagine. I do need to produce a square pulse for my research purposes with as little distortion as possible and to be able to vary the voltage and pulse width and so on. I am writing software using LabView to provide the trigger and control the system, so just after the amp to amplify the signal from the software.

Hope this makes sense! Am very grateful for the replies and help I have had so far, and am learning a lot about this! I've begun to source some parts (wire, connectors etc) and am trying to get an oscilloscope to test the signals that I'm getting once it's all assembled - very excited and really enjoying learning all this new stuff! :)