Suggestion Request: audio amplifier

[mio83]

Hello,

I would like to buy an affordable audio (pre)amplifier (0Hz to 100Khz) to use as a lab/test equipment (hobby level).
Ideally this unit would:

• be cheap
• have variable gain settings
• low noise figure
• be capable of amplifying weak signals (<100Khz) up to, say, 10/20V peak to peak (to a high impedance load), with good linear behaviour
• Have some basic low-pass (e.g., <20Khz) and high-pass filters (e.g., >100hz)

One think I'd like to use it, for example, it as audio section in a direct conversion radio.

QUESTION: Do you know of anything with these properties in the audio/music market, or stuff from aliexpress, or good old equipment now available for decent prices in ebay?


I have a nice HP 461A amplifier (linear and +40db) but it's max output is 0.5V (50ohm) and lacks the filtering options, and it is specified to work only on freq. >= 1Khz.

Thank you in advance for any suggestion.

[David Hess]

The old Tektronix AM502 1MHz differential amplifier meets those requirements.

[cdev]

Is that what an "instrumentation amplifier" does?

[mio83]

Thanks,

the Tektronix (and also some variants, like the Tek 7A22) looks exactly what I was searching for! 

EDIT: if you have other suggestions for more recent (and sufficiently cheap) differential amplifiers and/or instrumentation amplifier please let me know.

It would be desirable to have a standalone unit.

[David Hess]

The AM502 is the stand-alone version of the 5A22/7A22 but typically comes at a premium price.  Still, they can still be had for less than $200 in good condition.  The less expensive 5A22/7A22 plug-ins can be used as parts donors if necessary, or if you have a 5000/7000 series oscilloscope mainframe, the vertical output can be used with a 5A22/7A22 to supply a signal to another instrument.

There was a discussion thread here with a list of alternatives not too long ago.  Check my post which mentions Linear Technology application note 124:

https://www.eevblog.com/forum/testgear/preamplifier-for-noise-measurements/msg3074824/#msg3074824

[TimFox]

There are some nice Princeton Applied Research (PAR, later bought by EG&G) differential amplifiers with switchable high-pass and low-pass filters.  They can sometimes be found on eBay, for example https://www.ebay.com/itm/Princeton-Applied-Research-PAR-113-Preamp/193498466491?hash=item2d0d6848bb:g:gKsAAOSwx5le2Ud2 .  Note that I am not warrantying that particular listing, but I have two similar items.  I don't know if this model has NiCd batteries:  they can be a hassle for refurbishing, but a great help for ground loops.

[srb1954]

If you get an AM502 you will also need to purchase a TM500 series mainframe to plug it into. The mainframe supplies power to the AM502 and, with an opt. 2 mainframe, provides a rear panel connection for the amplifier output.

The TM500 series mainframes come in various sizes:
   TM501   1 plug-in slot in a bench case
   TM502   2 plug-in slots in a bench case
   TM503   3 plug-in slots in a bench case
   TM504   4 plug-in slots in a bench case
   TM515   5 plug-in slots in a travel case
   TM506   6 plug-in slots in rack mount case

The TM503 and TM504 seem to be the most readily available and give you several spare slots for future plug-ins. Check out TekWiki for the full range of plug-ins in the TM500 series.