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I would like to create a dummy load for this so-called "Deacy Amp":

deacy amp schematic

This is a low power germanium transistor / transformer driven push-pull amplifier for driving a small 4 ohm or 3.2 ohm speaker. After playing around with this circuit for a day it seems it is sensitive to the load. So my question is, what is the best way to load this amp in a way that will yield a complex frequency response. Ideally I would like to see a lot of middle frequency with notches and dips and peaks and so on.

I don't care if the load makes a little noise so enclosing a small 4 ohm speaker in a sound-resistant box is fine.

Another idea, that may be wildly misguided, is to take two 8 ohm speakers wired in parallel and mount them face-to-face possibly right against each other. How would this affect the frequency response of the amp's output? Would it increase the load or cancel it or magnify the complexity or have no effect or what? What if I used two slightly different speakers?

UPDATE 1:

Another idea is to use these "tactile transducers" which are effectively voice coils without the cones:

tactile transducers

I could attach these to plates of different sizes and weights sandwiched together with foam spacers to dampen the sound. The impedance of each plate would increase at it's resonant frequency and allow me to create a wide variety of frequency responses.

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  • \$\begingroup\$ Two speakers facing each other and closely mounted, would result in a noticeably higher effective impedance of the output. Each speaker is also effectively a microphone, so it would generate back-EMF corresponding to incident sound, opposing the drive signal. The arrangement will also add some frequency response maxima and minima due to acoustic resonance and magnetic coupling effects. This is true even if two mismatched speakers are used. \$\endgroup\$
    – Anindo Ghosh
    Commented Jul 11, 2013 at 5:57
  • \$\begingroup\$ I'm not sure what you are trying to ultimately achieve \$\endgroup\$
    – Andy aka
    Commented Jul 11, 2013 at 7:18
  • \$\begingroup\$ You need to be clearer about what you mean by "sensitive to the load" and "complex frequency response". In particular, why do you think the latter will help you diagnose the former? \$\endgroup\$
    – Dave Tweed
    Commented Jul 11, 2013 at 10:46
  • \$\begingroup\$ @Dave Tweed - This amp is "sensitive to the load" because the current in each transformer primary depends on the current in the secondary (the load). And the current on the output is frequency dependent when connected to a speaker whereas it would be flat with a 4 ohm resistor. So for example, if the speaker had a spike in impedance at 400 Hz. That would reduce the load at that frequency back to the collector of Q2. And because this is a class A amp, this would affect non-linear behavior in subtle ways and, more important, in not so subtle ways when pushed (which is what I intend to do to it). \$\endgroup\$
    – squarewav
    Commented Jul 11, 2013 at 13:52
  • \$\begingroup\$ A couple of misconceptions: First of all, this is definitely NOT a class-A amplifier. The output transistors are biased to operate (as near as I can tell) in pure class-B. Secondly, even if the collector load on Q3 and Q4 varies, this will have minimal effect on the rest of the circuit. \$\endgroup\$
    – Dave Tweed
    Commented Jul 11, 2013 at 17:10

1 Answer 1

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For starters, if your amp is sensitive to the load then any load that is different from the "actual load" will not give you the results you want. While an actual speaker (in the actual enclosure) will provide a complex load, any other speaker in any other enclosure will provide a different complex load. In essence, the graph of impedance vs. frequency will be different for the two loads-- in some cases significantly different. All of the notches and peaks in the graph will be at different places.

If your amp is sensitive to the load, then placing a significantly different load on it from the actual load will be no more valid than using a simple resistor load.

You should ask yourself if an amp should be sensitive to the load. Most people (including myself) would say that it should not be. But there are some niche applications where some other attribute is more important than audio clarity-- like low power or intentional distortion.

One important reason to not have an amp sensitive to the load is that the load will change over time. Speakers age. Components age. Sometimes speakers get damaged and replaced. If your amp is dependent on the speaker then it will not be guaranteed to work over time.

If your amp is sensitive to the load then the audio quality will also be load dependent. That means that you must test with the actual speaker. Additionally, the enclosure that you put the speaker in will change the loading-- so you must put the speaker in the final enclosure.

In my opinion, there is no point in making a dummy load for this amp. The dummy load will not match the final load, so you don't know if your amp will be stable or even work properly with the final load. You will also not know if it sounds good with the final load. Use the actual final load, or just a simple resistor load.

If you choose to use the final load, but you don't want to be blasting out noise all of the time then put the speaker (plus enclosure) into a sound-isolating chamber. The bigger the chamber the better, but it should be at least 5x the volume of the speaker enclosure. A "cheap" way would be to buy a used refrigerator and line the inside with rigid fiberglass panels. A used fridge can cost as little as US$150, or much less if it is broken.

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    \$\begingroup\$ Well it seems my question has completely missed the mark. The whole point is to exploit the "deficiencies" in the system because in my limited experience it is these deficiencies that are musically interesting. Like how some Germanium transistors have such a slow rise time, it dulls the otherwise obnoxious pick attack of a guitar. Or how the LFOs in the CS-80 synth waver whereas almost all synths that followed had very steady LFOs. Meaning this crappy circuit with a crappy speaker might be musically interesting. In fact, plugging a guitar into this circuit as-is sounds surprisingly good. \$\endgroup\$
    – squarewav
    Commented Jul 11, 2013 at 18:33
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    \$\begingroup\$ @ioplex I understood your question completely, but I have to write an answer that is suitable for you as well as others who are having similar issues but with different requirements. As stated in my answer, in my opinion, you should either use a resistive load or the actual speaker+enclosure. I do not believe that you will get useful results from attempting to make a more complicated dummy load. \$\endgroup\$
    – user3624
    Commented Jul 11, 2013 at 18:43

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