Marshall speaker reactance emulator

[vmaxvmax]

Hi I am attempting to build a vero version of a Marshall patent filed for a speaker reactance emulator. This is not a speaker emulator - just the interaction, resonance or reactance between a valve amp O/T and a speaker.

Is anyone happy to cast their eyes over my vero against the circuit in order to provide a double check?

Link is here: https://www.dropbox.com/s/ciqsadwtj4b41hi/Marshall%20Speaker%20Reactance%20Emulator.png?dl=0

Many thanks in anticipation...

[samhay]

Looks like the circuit is designed to run off a bipolar supply, so you will need to create a bias voltage and connect R1 and R27 to this rather than to ground. It will also need a capacitor on the output.

[vmaxvmax]

Thank you for your reply.

I only know enough about electronics to be dangerous!

I presume when yo say R27 you mean R5? Also, what about R2?

I am not familiar with bipolar supplies... I'm sure I can work it out though!

Did you get a chance to check the vero layout otherwise?

Thank you again!

[samhay]

Sorry - looks like I got the resistor names wrong.
Here is a schematic showing changes 'I' might make if I were to build it as a stand-alone stomp box - not sure what your intentions are.
I have added a voltage divider to generate a bias voltage (Vb), increased the input impedance to ~1M and added a some series and pull-down resistors to the input and output to make it place nice with others.
A simulation of the frequency response is below for anybody wondering what it does.

[vmaxvmax]

Wow! Great answer, thank you!

Most likely you fully understand the concept - but for anyone wondering about this circuit;

The purpose of the circuit is to emulate the reactance between a valve amp and a typical 4 x 12 Marshall style speaker cabinet. It does not simulate the speaker cab.

No doubt Marshall patented it to use in their S.S guitar amps.

This circuit is useful of instances where a line out is taken from the valve amp output transformer and a resistive dummy load is in place. Especially when the line out feeds a S.S. amp which then is fed to a traditional guitar speaker cabinet.

The line out is 'flat' in that it does not mirror the rising impedance at speaker resonance and from about 1khz upwards. This is one of the reasons a valve amp seems to have more warmth and bite compared to a S.S. amp without any emulation circuit.

The effect of this reactance is exactly as your graph shows! The frequency response of the circuit broadly mirrors the rising impedances.

Here is a great article explaining it: http://education.lenardaudio.com/en/14_valve_amps_7.html

Nowadays most digital amp simulators and cabinet simulators model this reactance.

This little circuit or similar is useful when running a valve amp hard into a dummy load and using  a line out from the O/T fed into a S.S. amp. This allows better control over volume whilst running the valve amp flat out and also allows time based effects to be placed after the valve amp but in front of the final S.S. amp.

It is much the same system the EVH used up until the 1990's.

It is useful to me because I like driving my 18 watt amp hard. The sweet spot is too loud for rehearsal and small gigs and too quite for larger gigs.

My 18 watt Marshall clone is run into a resistive dummy loud and a line out is taken to some outboard effects and then few into my Art SLA power amp which in turn is connected to a guitar speaker cab ( Marsall 4 x 12 or the Celestial G12h 30 already built into my 18 watt combo.

An alternative is to use a reactive dummy load (as opposed to a resistive load) that mimics the reactance between the valve amp and speaker cabinet. The downside of this is that the this type of dummy load is expensive, big, hot and generally not convenient to haul around!

Here is a great article from Aiken Amps on reactive dummy load design: http://www.aikenamps.com/index.php/designing-a-reactive-speaker-load-emulator

I hope this helps anyone who wants to know.

Once again, Samhay, thank you for your most helpful answer and circuit!

[tca]

Interesting. I'll have to try this.

Cheers.

P.S. (edit)

I think it does more than simulating the speaker reactance, probably the complete frequency  rough response of the power amp and the speaker taken conjunctly.

[samhay]

No problem. Happy to look over a revised vero layout too if you like.

[teemuk]

I think that little circuit made a debut in this:
http://www.amptone.com/marshallse100.htm

Basically, this one employed Marshall's reactive "PowerBreak" attenuator / dummy load for speaker level inputs so for that part there was really no need for any "emulations". However, signal to "Line In" -jack does not pass through the PowerBrake so jack switching incorporated within the Line In jack switches this little circuit stage in whenever that signal source is in use.

After that the signal advances to much more complex set of filters that actually try to mimic frequency responses of different speaker systems.

So yes, this little circuit only simulates how non-linear speaker impedance changes voltage gain (and hence frequency response) of an amplifier with high-ish output impedance. "Cab simulations" are a different deal.

[vmaxvmax]

No problem. Happy to look over a revised vero layout too if you like.

Thank you - I am not always 100% confident on designing vero's... So If you are happy to give a second look over I would be very grateful.

I shall probably get around to it this weekend or at the beginning of the week.

Thanks again,

[vmaxvmax]

No problem. Happy to look over a revised vero layout too if you like.

Here goes: https://www.dropbox.com/s/pg5d74tjx72ytt4/Marshall%20speaker%20reactance%20emulator.pdf?dl=0


I would be grateful for any second set of eyes on this vero!

Thanks in advance

[vmaxvmax]

HI Samhay,

I have PM'd you - but for anyone else, I plan to use a charge pump in order to get as close as possible to the max voltages in order to preserve headroom. The Tl072 can take up to 18 v.

I hope this little project might be of use to others.

There are  a few patents for this type of emulator but nothing available in a small package without speaker cab simulation. I'm sure there must be others like me who don't need the speaker cab simulator if they are using guitar cabs!

[mth5044]

The TL072 can take +/- 18V, so -18V to +18V or 0V to 36V. I can't see anything in Dropbox, so I can't help you with the layout.

[samhay]

If you are going to use a charge pump and are happy with 18V headroom, then I would use it to create a +/-9V bipolar supply and use essentially the original circuit.

[vmaxvmax]

The TL072 can take +/- 18V, so -18V to +18V or 0V to 36V. I can't see anything in Dropbox, so I can't help you with the layout.

I think you may have to wait for the Dropbox file to load up. The link works for me...

[vmaxvmax]

If you are going to use a charge pump and are happy with 18V headroom, then I would use it to create a +/-9V bipolar supply and use essentially the original circuit.

Did you get to look at the vero?

[mth5044]

The TL072 can take +/- 18V, so -18V to +18V or 0V to 36V. I can't see anything in Dropbox, so I can't help you with the layout.

I think you may have to wait for the Dropbox file to load up. The link works for me...

I can't access dropbox on my laptop or phone, there have also been other people having trouble accessing it. Imgur is a popular host that almost everyone can see. I borrowed a computer to see if I could access it on a Mac and got in. Vero looks fine to me.

[samhay]

If you are going to use a charge pump and are happy with 18V headroom, then I would use it to create a +/-9V bipolar supply and use essentially the original circuit.

Did you get to look at the vero?

Have taken a look* over the vero layout and don't see any glaring issues.
However, as I said earlier, I would do things differently if I was going to use a charge pump.

*I have had a few cheeky pints after work this evening and thus probably can't be trusted.

[vmaxvmax]

The TL072 can take +/- 18V, so -18V to +18V or 0V to 36V. I can't see anything in Dropbox, so I can't help you with the layout.

I think you may have to wait for the Dropbox file to load up. The link works for me...

I can't access dropbox on my laptop or phone, there have also been other people having trouble accessing it. Imgur is a popular host that almost everyone can see. I borrowed a computer to see if I could access it on a Mac and got in. Vero looks fine to me.

Thank you for looking! - I'll look into Imgur.

[vmaxvmax]

If you are going to use a charge pump and are happy with 18V headroom, then I would use it to create a +/-9V bipolar supply and use essentially the original circuit.

Did you get to look at the vero?

Have taken a look* over the vero layout and don't see any glaring issues.
However, as I said earlier, I would do things differently if I was going to use a charge pump.

*I have had a few cheeky pints after work this evening and thus probably can't be trusted.

A few 'cheeky' pints can be quite useful...

I did see your post earlier. I'm not sure what you mean though... Don't forget, I am an imbecile...

[vmaxvmax]

Thinking further on this, do you both mean use the original Marshall circuit and just supply +v to pin 8 and -v to pin 4 (instead of earth) and all other earth connections as in the original schematic?

I had thought to use a 33v charge pump to provide hopefully around +/- 15v... I had assumed that the current draw for this would be within limits.

Am I on the wrong track?