[Folsom]

Very nice indeed!

Is the AC shield grounded? It should be.

[Odal3]

 

[limits]

Great job, George!! Enjoy!

[Peter J]

Here's another (almost) finished amp, need to install mica insulator (not wanted silicon one) and some feets that will have
double role here: They are also used for fastening sub-chassis carrying Alu bracket(s) / PCB boards / heatsink etc.
Enclosure is 220 x 100 x 300 mm WHD.

George

Wow, nicely done George. It appears to be really clean and well thought out...something to aspire to. You've set the bar way up there and it really appeals to the detail freak in me.

[nik.d]

Thank you all. Special thanks for PeterJ who's marvelous and well documented speaker building is really, really inspiring.

@Folsom
Yes Jeremy, whole enclosure including 2 brackets is/are grounded - pic #4, green wire -> GND @ Neutrik powercon -> lower screw to enclosure's back plate. The only non grounded objects are heatsink
and this nickel plated brass tube for AC 230V wires. Removing heat shrink 'collars' (short, red one's) will make contact with both aluminum brackets. Re-thinking, adding a GND wire to tube beginning,
removing 'collars' and this way I have 1 point grounding, not via bottom plate / profiles. Hmm, not bad and very simple to apply...

[mlundy57]

I think I remember reading somewhere that the gain for this amp is 32dB. Is that correct?

What is the input sensitivity?

Thanks,

Mike

[Folsom]

http://www.radiotechnika.hu/images/TDA7297.pdf

[mlundy57]

http://www.radiotechnika.hu/images/TDA7297.pdf

Thanks

I saw where the gain was listed but couldn't find, or didn't understand what I was looking at if I did, where the input sensitivity was listed.

Mike

[Folsom]

Gain is 32db. Input sensitivity I think is 1.1v

[mboxler]

Gain is 32db. Input sensitivity I think is 1.1v

For my own education...

32db voltage gain equates to almost 40 times input voltage.
With 18 volt PS, the max output voltage would be 12.7 volts RMS.

12.7 / 40 = .3175 volts.  Seems like any input over .3175 volts RMS would clip the outputs.

Am I missing something?

Mike

[mlundy57]

For my own education...

32db voltage gain equates to almost 40 times input voltage.
With 18 volt PS, the max output voltage would be 12.7 volts RMS.

12.7 / 40 = .3175 volts.  Seems like any input over .3175 volts RMS would clip the outputs.

Am I missing something?

Mike

I thought input sensitivity was the minimum amount of voltage required at the RCA input jacks (from the source) needed to drive the amp to to full power and that this was something different than voltage from the power supply.

However, with my less than basic understanding of the physics behind electronics I could be way off base.

Mike

[mboxler]

I thought input sensitivity was the minimum amount of voltage required at the RCA input jacks (from the source) needed to drive the amp to to full power and that this was something different than voltage from the power supply.

However, with my less than basic understanding of the physics behind electronics I could be way off base.

Mike

The power supply voltage (and current) dictates the maximum output power.  These are internally bridged amps...1 amp for each + speaker out and 1 amp for each - speaker out.  Each speaker out can swing from 0 volts to the power supply voltage.  The signal to the - amp is inverted, so it will swing the exact opposite of the + amp.  With no signal, there is 1/2 PS voltage on each speaker out.  With an 18 volt PS, you should measure 9 volts between ground and either speaker out.

In reality, the amp will distort if the output signal is amplified too close to 18 volts peak to peak, but hopefully you get the picture.  If the signal is amplified more and 18 volts , the signal is clipped...a bad thing.

As you can see, a PS of less voltage (say a 12 volt battery) will lower the maximum output voltage, and therefore lower the maximum output power.

Mike

[Folsom]

The fact that it is bridged (balanced output) means that your calculations (if correct) need to be doubled for input sensitivity.

Also it's been tested to not clip a fair bit higher than 15w (24w into a 6ohm resistor).

[mlundy57]

OK so two different issues. Instead of clipping, I'm looking at component matching.

Specifically whether a passive preamp (tube buffer) with 0 gain will work for switching and volume control with a source that has an output of 2V or whether an active preamp with additional gain and a higher output voltage will be needed.

I take it from Jeremy's response that with 32dB of gain and input sensitivity of 1.1V the passive tube buffer I currently use will also work with this amp.

Mike

[Folsom]

It's not passive if power goes to it.

I'd recommend zero gain buffers for best match.

If it's truly passive (no tube or transistors) then 10kohm is the highest rated volume attenuator I would use.

[mlundy57]

It's not passive if power goes to it.

I'd recommend zero gain buffers for best match.

If it's truly passive (no tube or transistors) then 10kohm is the highest rated volume attenuator I would use.

Ok not passive then. It's a Dodd Audio Battery Powered Tube Buffer with 0 gain

Mike

[Folsom]

Should work good. The attenuator impedance won't matter with the buffer following it.

[mlundy57]

Should work good. The attenuator impedance won't matter with the buffer following it.

Thanks

[Folsom]

Someone emailed me, but they're address doesn't work when I try to reply. I assume it's someone on here that typed their address incorrectly. In case whomever it was, is wondering.

[slefley]

Gain is 32db. Input sensitivity I think is 1.1v

Another question on input sensitivity - I am using a LampizatOr with volume control to drive my power amp directly.  Recently I tried a Fleawatt amp but because if it's high sensitivity I couldn't get the Lampi volume control above -50bB before the it got too loud.  Unfortunately the Lampi volume control works much better at the high end of it's range, so I couldn't use the Fleawatt in my system.  Do you expect that I'll have the same issue with the Folsom 7297?