Amp question - Purpose of a choke

[murraymurray]

Could someone give me an idiots explanation of what a choke does in amps?
not all amps have them right?

[Frank]

It's used to smooth out "ripple" currents in the high voltage DC supply to the power amp. As it's usually a fairly heavy and expensive part of the amplifier, some manufacturers eliminate it from the amp design and use high-value capacitors instead.

If your amp has a choke then it'll be fitted just after the first power supply filter capacitor, it's the thing that looks like a smaller version of the power transformer. Internally it's like a HUGE non-magnetised pickup - thousands of coils of wire wrapped around a lump of metal that resists fluctuations in voltage so if you pass in high voltage DC with a bit of noisy AC ripple current, the output will be slightly lower voltage DC (due to the choke's internal resistance) minus the noisy wobbly ripple current.

[Dmoney]

As it's usually a fairly heavy and expensive part of the amplifier, some manufacturers eliminate it from the amp design and use high-value capacitors instead.

sure you don't mean a large value and power rared resistor?
A an example of this is the Yamaha T100 which was originally designed with a choke, but then yamaha replaced it with a resistor.

[Frank]

A resistor on its own provides no ripple filtering. You've got to put a capacitor after it or all it'll do is drop the voltage.

[Dmoney]

but if you just replaced the choke with a resistor, wouldn't you normally have a filter cap each side?
im just trying to visualise it. if you pulled the choke in a jcm800 and replaced it with a cap, you'd have rectifier, filter cap, capacitor, filter cap right?

[Frank]

Yes you're right, there's a filter cap after the choke so it would still do the ripple filtering. I'm just wondering how it would affect other voltages throughout the amp though, surely it'd reduce the voltage to the screen grids and to the rest of the amp?

If it was originally designed with a choke, replacing it with a resistor just seems like penny-pinching possibly at the expense of performance

[Dmoney]

Yes you're right, there's a filter cap after the choke so it would still do the ripple filtering. I'm just wondering how it would affect other voltages throughout the amp though, surely it'd reduce the voltage to the screen grids and to the rest of the amp?

If it was originally designed with a choke, replacing it with a resistor just seems like penny-pinching possibly at the expense of performance

yeah in the case of that yamaha is was exactly about penny pinching. i think they took the original soldano design, replaced the choke with a resistor and messed with the reverb to save money.

i don't fully get it, but i think in high output amps, youd get that voltage drop like you said, and probably excessive sag? im kinda guessing. it probably wouldnt like massive palm muted chuggyness.

[Frank]

I hate it when penny-pinching accountants get hold of the designs and start crossing out the bits they think are eating into the profit margin.

Having said that, I own a 1983 strat.

[HTH AMPS]

you typically get a bigger voltage drop across (sag) with a resistor in the Pi filter versus using a choke.

I personally like chokes as they give better filtering/ripple and a tighter feel.  I wouldn't say they're expensive either (a common myth), a typical choke for a Marshall 100w amp will cost in the region of £25.00 if they're specifically marketed as a 'repro'.  However, if you know the specs, you can use a Hammond equivalent for half that price.  Even Mercury Magnetics only charge US$30.00 for their chokes.

If you want a good idea of the 'feel' difference between a resistor and a choke in a Pi filter, hook them up to a switch.  

 

[Frank]

yes but tell an accountant you can shave £15 or £25 off manufacturing costs and he'll jump at the chance

[jpfamps]

Great sounding amps have been built with either choke or resistor screen filters, so I wouldn't discriminate against an amp on that basis (although of course the bean counters would prefer the latter......)

A choke will give better filtering, lower voltage drop and the voltages will hold up longer under heavy loading before dropping.

A power resistor will usually be much larger than the DC resistance of a choke, and so will drop more voltage both at idle and under load, and hence will have more sag.

The voltage drop across the resistor at idle means that the screens are running at a lower voltage than the B+ which is beneficial for valve reliability.

I would have to say, as a general rule, the amps I like with the best clean tones have choke filtered screen supplies.

For single-ended amps with lower current draw using a choke to filter the B+ is very worthwhile as SE amps are don't reject hum on the power rails very, so they benefit from a better filtered supply.

[HTH AMPS]

For single-ended amps with lower current draw using a choke to filter the B+ is very worthwhile as SE amps are don't reject hum on the power rails very, so they benefit from a better filtered supply.

this is my feeling too, but the general consensus on tinterweb forums is that this is a waste of time.  imo, its more beneficial with SE than with PP amps (though I prefer chokes in general).

I've got a stereo rack power amp with resistors used in the Pi filter and am planning on modding one side with a choke so I can A/B the tonal change (and also give a broader range of tones in general - there is more modding planned)