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Soak History, Randall Aiken's Soak Circuit

Re: Aiken Attenuator/Load Randall Aiken

Design equations and a schematic of the Aiken attenuator design

>Re: Power Attenuators/Soaks. (From here you can read the entire thread.)

Following are some of my favorite, conceptual, less-technical paragraphs from the posting. -- Michael

>> Randall Aiken wrote:
>> "A speaker presents a varying impedance load to the amp. The stated
>> impedance is usually measured at 400Hz, and can vary widely over the
>> frequency range. Speakers generally have a large resonant peak that
>> can be as much as 5 times the rated impedance, or even higher. The
>> impedance also starts to rise upward following the trough impedance at
>> 400Hz. This rise can occur at varying frequencies and have a varying
>> rate, depending upon the speaker and its enclosure.

Aiken Speaker Simulator Circuit

>> Here is a copy of my design for a circuit that will simulate the
>> impedance variations you get with a 16 ohm speaker cabinet. The real
>> speaker will probably vary depending upon how hard it is driven, due to
>> motional restrictions of the cone, which this circuit will not do.
>> This circuit will make the amp react to the load unlike pure resistive
>> dummy loads, but if you want to tap off the input and send the signal
>> to a board, or other equipment, you will need to attenuate it with a
>> voltage divider, and low-pass filter it to simulate the frequency
>> response rolloff of the amp.
>If you want to add a line out feature for recording, tap off the input
>of the circuit; that is, at the junction of the amplifier output and R1.
>Use a resistive divider to lower the signal level. Note that this
>circuit provides a load impedance that approximates a speaker, but it
>does not provide a frequency response that approximates a speaker. If
>you want to use such a device as a silent load for recording, you will
>have to add further lowpass, highpass, and bandpass filtering that
>approximates a speaker frequency response for it to sound good.

Using the DejaNews link above, you can see this other posting in the thread.

Somone wrote:

>Randall Aiken,

> I would be very curious to know your opinion of the THD HOTPLATE, and its supposed "frequency compensation" circuitry, its "bright and deep" switches and its noise control circuit (light bulb). It seems really well-built. Kolbe doesn't seem to think to highly of the THD unit, claiming it is essentially "non-reactive"...(the Air Brake is completely resistive I understand.) Is the "reactive" element--i.e.impedance curve accuracy compared to real speakers-- essential to full dummy loads, but not so important for attenuators where a lot of energy is still going to speakers??

I'll let you know my opinion of the Hotplate as soon as mine arrives. Like yourself, I bought mine on the recommendation of people who owned them.

I have several other commercial units, including the Altair Power Attenuator (the unit that started it all way back in the late '70s), a Scholz Power Soak, and a Marshall Power Brake. From what I've heard from people who have compared them, the general consensus is that the THD unit sounds best, followed by the Power Brake. I believe the Air Brake is too new to have generated many comments, at least among people I've heard from. [the guitar salesman at the used guitar store said the Hot Plate sounds better than the Marshall. At extreme attenuation, the Hot Plate sounded just as awful and fizzy and un-amp-like as any attenuator or load I've heard. - Michael]

As to the importance of simulating the impedance characteristics of a loudspeaker, that is an interesting subject. I find that it really depends upon several factors including the amount of attenuation, the room characteristics, and the amplifier output section itself.

I prefer the sound of a pure resistive attenuator at very low levels (playing along with the stereo at night). I prefer the sound of the reactive load at very small amounts of attenuation (loud live gigs), except in very live rooms, where it sounds rather shrill. The purely resistive load can seem muffled or flat in some places. I have even gone as far as running two units in series, one reactive, and one resistive, and adjusting the levels of each for the best sound (this is also a great way to get really low volume levels for apartment dwellers and the like).

I have read postings by some people who claim a purely resistive load will damage a tube amplifier whereas a reactive load will not. I don't recall the exact claim, but it wasn't backed up by any theoretical explanation, just conjecture. The pure resistive load should be safe for the amplifier; I run my amps full up into a load resistor on the bench and see no such problems; however, I have noticed a reactive load will sometimes cause a feedback amp to oscillate (another good reason for no negative feedback loop on the output stage...).

[I had squealing problems with the Silent Speaker reactive attenuator, with my Marshall 4010 1x12 50 watt combo amp. - Michael]

> How do your designs relate to existing attenuator/load products?

I wouldn't really call my designs "mine", as they are nothing more than modifications to the well-known speaker simulation circuitry as described in RDH4 and other places.

The circuits I posted previously were simply my circuit values to attain desired resonant frequency points and peak impedance variations, along with high frequency impedance limiting and DC/low frequency impedance values slightly lower than the midband impedance as is common in a standard loudspeaker.

I initially designed mine for use in testing amplifiers into a reactive load at full volume without blowing the neighbors away.

-- Randall Aiken

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