Trying to Understand Resistors & Lpads with Bass Drivers

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Tarkers
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#1 Trying to Understand Resistors & Lpads with Bass Drivers

Post by Tarkers »

I'm trying to get to grips with the effect of resistors and Lpads on bass drivers and particularly how they may affect the T/S parameters.

My understanding (which may well be faulty) is as follows:
1. A single series resistor on a bass driver will attenuate output but at the same time increase the Qes and therefore Qts meaning increased system Q (sealed box) for the same size enclosure.
2. A single resistor in parallel with the driver will reduce the Qes and therefore Qts meaning reduced system Q for the same sized sealed box.

Firstly, are 1 & 2 correct?

Secondly, suppose that one wanted an Lpad to attenuate an 8 ohm bass driver by 6db. This would require an 8 ohm parallel resistor and a 4 ohm series resistor. If the parallel resistor exceeded 8 ohms, would it have a similar effect on Qes as a single parallel resistor as described above? Similarly, if the series resistor exceeded 4 ohms, would this have a similar effect on Qes etc as a single series resistor?
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IslandPink
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#2 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by IslandPink »

Good questions, will have to think on LPads, but I think I can comment on 1. and 2.

Point 1. is correct, the Qes increases. I've seen the effect of this on sims and heard the effects too. The amp will 'see' a higher resistance overall on the speaker too - this will change the load-line ( to be flatter ) in a valve amp - which may or not be a good thing. SS amps won't mind much.

I don't think point 2 is correct. Looking from the driver's point of view, it still has the same internal resistance ( Re ) and it still 'sees' the amp direct, ie. having the normal low driving impedance ; so I do not believe Qes is altered. The amp will 'see' a lower overall impedance though - that of the speaker and the resistor in parallel. If the amp has quite a high output impedance ( eg. more than a couple of ohms ) then there could be an effect, though, since I guess the electrical damping the speaker sees will be set by the amp in parallel with the resistor. Usually the amp is by far the lowest R in the pair though.
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DSJR
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#3 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by DSJR »

Might it not be considerably better to bi-amp and slightly attenuate the amp feeding the bass driver. I thought the object was for the amp to look as directly to the speaker coil as possible with nothing in between (preferably) except a length of cable. That way, the amp isn't being 'disconnected' from the driver due to the resistor(s) and can supposedly 'see' it better.

(I know, I know, I don't have a degree in electronics, but the above is how I've had it explained to me I think, by people who claim to have studied this in detail)...
Tarkers
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#4 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by Tarkers »

I fully agree that a separate amp is the way to go if possible and the fewer bits and pieces you have in the signal path the better. I was thinking of a situation where (a) a second amp is not practical and (b) where one is trying to balance a multi-way system where the bass driver output exceeds that of the tweeter department. As a corollary I was hoping that if one had a bass driver where the T/S parameters indicated too large a sealed box for sensible QTC, a small parallel resistor would have the effect of reducing Qes and therefore Qts.

I should add that I'm trying to help out a fellow DIYer with particular drivers to hand as opposed to drivers I would go out and buy.
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#5 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by chris661 »

Connecting a parallel resistor will simply increase current draw from the amp. There'll be no change in the driver's T/S parameters. Consider this - when viewed from the driver, the amplifier is basically a short-circuit. Putting a couple of ohms in parallel won't change the 0.001ohm resistance the speaker is seeing.

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#6 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by IslandPink »

On the L-pad question, a typical design is like this :
l-pad3.jpg
l-pad3.jpg (6.98 KiB) Viewed 7778 times
- often used for treble sections hence the high-pass ( cap ) here.
The idea is to allow level adjustment while keeping the impedance of the treble section as seen by the rest of the crossover, and the amp, constant.
This is achieved , but the amount of series R between the amp and the speaker here is not constant, so the Qes will vary depending on the level you set.
In a tweeter this will probably not matter much, but if you try it on bass then the same effects will happen as with a resistor in series ; however the amp will be loaded better.
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#7 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by Tarkers »

Thanks chaps. That all makes sense and also explains what might happen if you get the L-Pad resistor values out of line with what's needed for a particular DB cut.
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#8 Re: Trying to Understand Resistors & Lpads with Bass Drivers

Post by RushPCB »

IslandPink wrote: Thu Oct 05, 2017 1:10 pm Good questions, will have to think on LPads, but I think I can comment on 1. and 2.

Point 1. is correct, the Qes increases. I've seen the effect of this on sims and heard the effects too. The amp will 'see' a higher resistance overall on the speaker too - this will change the load-line ( to be flatter ) in a valve amp - which may or not be a good thing. SS amps won't mind much.

I don't think point 2 is correct. Looking from the driver's point of view, it still has the same internal resistance ( Re ) and it still 'sees' the amp direct, ie. having the normal low driving impedance ; so I do not believe Qes is altered. The amp will 'see' a lower overall impedance though - that of the speaker and the resistor in parallel. If the amp has quite a high output impedance ( eg. more than a couple of ohms ) then there could be an effect, though, since I guess the electrical damping the speaker sees will be set by the amp in parallel with the resistor. Usually the amp is by far the lowest R in the pair though.
Very useful input! I must agree with your threat.
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