Recommendations for audio amp

[W8ACR]

While doing some testing on my SB610 scope to see if I could get it to display a "real" trapezoid pattern, I ran some audio from my HP200 into the W2IHY audio amp/equalizer, and I think I cooked one of the IC's with too much audio power. There are five IC's in there and I have already ordered new ones from Mouser. (By the way, IC's are really CHEAP these days - five IC's cost a total of $3.45!) If replacing these IC's doesn't fix the problem, I will need to get a new audio amp since my Heil mike doesn't have enough intrinsic output to drive the audio deck of my homebrew transmitter. I could build myself a little preamp using a 12AX7, or buy a commercially built unit. I'm looking for suggestions on what kind of audio amp to buy. The mike is a Heil Classic and has an output of -72dB at an impedance of either 600 Ohm or 50K ohm. The input of the audio deck is a 6SJ7 with 2meg resistor on the grid. I don't need a fancy system with processing/compression/equalizing or any of that fancy broadcast hoohaw. I just want something that will give me 50dB gain or thereabouts with minimal distortion. What do you guys think?

Thanks, Ron W8ACR

[WBear2GCR]

you want to go from mic level to line level??

I would consider a buffer on the mic side into a step up transformer, followed by either nothing or another buffer depending on the specific requirements.

In other words, get ur voltage gain from the transformer, current gain from the buffer(s).

Since you would need to drive a big step UP, that is likely to look like a 600 ohm : 20kohm or higher ratio. So you need to be able to drive the input side with enough current to cover the 600 ohm winding. That's a buffer.  Since you have a high Z on the other end, you can likely skip the buffer there. Done. And probably no RF issues, bullet proof too.

Choose ur transformer, an old UTC or Altec or newer, like Cinemag, Lundahl or Jensen... The buffer can be some paralleled JFETs or a purpose made IC, etc...

                _-_-bear

[steve_qix]

It sounds as if you just need a mic preamp.  You can build one of these very easily.  Either use a low noise op-amp (there are so many) - even a  TL072 [I said this for Bear's benefit  ].   I built my own mic preamp/mixer using TL072s.  The unit also provides standard phantom power, if needed.

I don't remember the specs for phantom power right off (they are on the Internet - wikipedia, I believe is a good source), but it is some voltage (I think 48 volts) with a source resistance of some very reasonable value.  Since phantom power is provided against a balanced input, configure the op-amp as a differential amplifier.  The phantom power is common mode with respect to ground (both inputs carry the phantom power with respect to ground).  I used a couple of 100uF DC blocking capacitors to isolate the phantom power from the op-amp inputs and of course protected the op-amp inputs with diodes running to the power supply voltages (+/- 12V) to prevent damaging input surges, etc. when unplugging and plugging in mics.

This works just fine for unbalanced as well (phantom power switched off).

[KX5JT]

This works well.... 30 bucks

http://www.musiciansfriend.com/pro-audio/art-tube-mp-studio-mic-preamp



Features

    Provides Superior Preamplification for: Microphones, Instruments and Line Level Sources
    Analog VU Meter
    OPL Output Protection Limiter
    Hand-Selected 12AX7A Vacuum Tube
    Variable Input and Output Gain Controls
    Provides over 70 dB of Gain
    +48 Phantom Power Switch
    Phase Reversal Switch
    XLR and 1/4" Inputs and Outputs
    Portable, All-Steel Construction
    Excellent as Tube DI
    Built-in Brickwall Limiting
    20dB gain switch

[steve_qix]

You know what's amazing?  That such a thing could be built SO CHEAPLY !!??!!  When we were manufacturing the mod monitors, the EMPTY CABINETS (no parts - just the cabinets) were $70.00 each, and I did a LOT of shopping around, and did everything possible without sacrificing quality to keep the price down....

Just amazing about the price.

[WBear2GCR]

Steve,

Just buy a FULL shipping container of whatever you want from China... low priced then!

Yeah, Steve, I use the TL072/74 all the time!   (for Steve's benefit!)
...I guess one could say that he and I have a running gag going on that part... nevermind.

                      _-_-bear

PS. "hand selected" - Yep! A chinese girl reached out to a bin with HER HAND and picked one!

[ke7trp]

That little ART preamp sounds great!  I had one,  I also had the Presonus Tube pre.  Same deal sounded great!  Both are nice as you can flip the phase back and forth. 

I run the ART Voice channel Here.  A fantastic voice channel at a great price.  I got rid of a huge stack of audio gear.  I just run the mic into the voice channel, Into my DAP310 processor and into the Transmitter.  Must have removed 50 ft of cabling and mess from my shack.

C

[Steve - K4HX]

I've used the ART box too (similar to the one shown in KX5JT's post but without the meter). They work well. I can't believe they are selling for only $30!

[K5UJ]

<<Hand-Selected 12AX7A Vacuum Tube>>

What's the B+ on the 12AX7A?

[ke7trp]

I dont think the tube is used in the traditional manner. Its 9vac on the input side.  I read somewhere that tube amps like this where really a hoax as its used for a diode or something.  I do not have the schematic so I do not know. However, The little pre does sound nice.

C

[K5UJ]

I dont think the tube is used in the traditional manner. Its 9vac on the input side.  I read somewhere that tube amps like this where really a hoax as its used for a diode or something.  I do not have the schematic so I do not know. However, The little pre does sound nice.

C

Okay thanx that's vy interesting--well the sound is what counts.

[KX5JT]

I dont think the tube is used in the traditional manner. Its 9vac on the input side.  I read somewhere that tube amps like this where really a hoax as its used for a diode or something.  I do not have the schematic so I do not know. However, The little pre does sound nice.

C

Okay thanx that's vy interesting--well the sound is what counts.

Not a diode but a "plate starved amplifier".  Really all the work is done by solid state but they do pass the audio "through the tube" to give it "tube audio characteristics and modeling".  Whatever.  Bottom line is it does work.  It sounds good.  You can invert the phase at the push of a button.  It accepts hi-z or lo-z and puts out hi-z or lo-z.  Not bad for 30 bucks.