Passive Subs - What Amp?

Mark Ward

Prominent Member
Sep 12, 2001
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Seriously considering 2 X SVS 20-39CS-Plus subs now after starting a big heated debate in the "CHT or Servo15?" thread :blush:

These SVS passives would need some serious amplification and SVS recommend their Samson S-1000. What would be a viable power amp alternative available in the UK?

It was a toss-up whether to post this is Speakers & Subs or Amps forum but since it is specifically Subwoofer related this seems to be the most appropriate forum I guess.

Having discovered Tom from SVS frequents the forums I thought I'd ask this here. Please feel free to reply even if you're not Tom:)

I have 4 Harmon/Kardon PA2400 Power Amps. I could spare 1 or 2 to power passive subs. I'd only be interested in doing this if they would perform VERY well.

Harmon/Kardon PA2400 Signature Series
Continuous Average Power
Per Channel (FTC), from 20Hz to 20kHz
both channels driven.
8 Ohms: 120 Watts @ <0.08% THD
4 Ohms: 170 Watts @ <0.08% THD

Bridged (FTC), from 20Hz to 20kHz, Mono
8 Ohms 340 Watts @ <0.15% THD

I was thinking perhaps one of these bridged per SVS 20-39CS-Plus sub? Are the subs 8 Ohms?

HK PA2400 Specs
Dynamic Power (IHF, 1kHz tone burst)
8 Ohms: 150 Watts
4 Ohms: 270 Watts
2 Ohms: 440 Watts

High Instantaneous Current Capacity (HCC): +/- 100 Amperes

Negative Feedback: 12dB
Power Bandwidth @ half-rated poutput, 8 Ohms: <10Hz - >100kHz

Frequency Response @ 1Watt (+0/-3dB): 0.1Hz-250kHz
Slew Rate: 280 Volts/uSec
Rise Time:1.2 Volts/uSec
Transient Intermodular Distortion (TIM): Unmeasureable
Damping Factor: 120
Signal:Noise Ratio (ref rated power,A-wtd): 120dB
Input Sensitivity/Impedance:0.8 Volts/33kOhms


The SVS CS 20-39 present a 4 ohm load.

It is on their site.

Here is a small section for you...

So what kind of amp will power one or more CS subs? Nearly anything. Old stereo amps, old surround receivers, mono-block amps, you name it. SVS’s are tremendously efficient since they require no power robbing equalization to run low. 100 watts delivered to the sub is enough to fill your HT with amazing bass. One of the reasons we offer the CS line is so that some of those venerable Adcoms, Soundcraftsmen, Pioneer, Marantz and other nice amps and receivers get a second lease on life!

SVS’s present a nominal 4-ohm load but this is very dynamic, actual impedance or draw will usually be much higher than this since the impedance varies dynamically with the program intensity and frequency of the bass. We’ve run extensive user tests with a pair of CS subs driven by $80 stereo receivers with no difficulty at all. Used pro-style amps are another source of good clean power too, and can be bought for as little at $150. If your amp/receiver isn’t rated in its manual for 4ohm loads you do need to exercise some preliminary restraint. Do some testing of your own to make sure your amp does not get hot or express any other signs of instability (distortion, popping, etc.) at high levels. This is unlikely but you must be certain the amp is up to the task. Even amps rated at "only" 100 watts into 8 ohms can power an SVS with tremendous results however so it's worth a bit of trial on your part. Needless to say, if your amp DOES get hot or produces audible distortion you need to find something else suitable. Please note that very few old receivers or amps allow their output channels to be "bridged" so plan on using one channel for each SVS (one channel can go unused if is a stereo amp and you have only one SVS). Anything from 100 to 300 watts is perfect for one of our subs; more power will give you little increased, clean and flat bass though within recommended calibration levels it will do little harm either.
micb3rd - You're from Bedford and you also work for SVS?

From what you say I could power 2 Passive SVS subs from one of my H/K amps with good results?

They are bridgeable but they will offer 4 Ohms: 170 Watts @ <0.08% THD per channel if used separately.

This sounds like good news to me. One thing.. What does the Samson amp SVS sell offer over a regular 2 Channel power amp?

Things are looking up.

Hi Mark

The SVS '+' line of subs need 500W for maximum performance and you'll need maximum in that cathedral of a room. :)
You amps are a bit under powered for the application IMO.

You need an amp with plenty of 'grunt' hence the pro amps, excellent power at reasonable cost.

You can get Samson amps in the UK, try here.

QSC, Crown & Crest all make quality pro power amps.
You may even find some great second hand bargains.
There's plenty of pro equipment dealers in the UK.

Thanks Ramius! I thought it would be a bit too easy to simply use my H/K's.

I'll do some research on the QSC, Crown & Crest professional amps. The the Samson may be the easy solution, not least because SVS recommend and sell them. Buying electronics in the UK is more comfortable as warrantee issues are so more straight forward. However, if Samsons have an International Warrantee and SVS can supply 220-240V version competitively (Duty & VAT factored in) I'd kind of like to buy from them if I go the SVS route.

Anyone selling a second-hand Samson 1000? :)

How Important is the quality of the speaker wire with passive subs BTW? I'll have to factor some serious runs into the cost.

Try and place the amps nearish to the subs and use a longish interconnect if possible. Cheaper and better especially in your sports hall!
Will probably work very well - as SVS say the subs require no EQ, then the power should be plenty.
Even a 6dB EQ boost at say 30Hz requires 4x the power from the amp at 30Hz (and it's not just 30Hz which will get boosted - depending on the design, you could be +5dB at 28/32Hz, +4dB at 26/34Hz, +3dB at 24/36Hz and so on - this takes a lot of amp power, and EQ boosts of more than 6dB are not that uncommon. If you have a 1000W amp, itmay not be an issue, but with 100W, you can start to run out of steam before you know it)

One possible hiccup though would be how you control the volume - you'll only have the sub level control on the AV amp, and it might not offer the necessary control range.
Still, knocking up a DIY level control for the subwoofer line level wouldn't be too difficult!

Another might be the respone of the amp itself, though you'd have to try it. As it's not specifically designed to power a subwoofer, it's low end frequency response may well be filtered to protect it.
However, IMO 20Hz should be enough for anybody :)

Finally, without a phase control, positioning may be a touch more difficult.

If you don't fancy it at the end of the day, take a look at the sub amps at
A couple of their 300W modules and a bass controller would only set you back about £250 delivered. Build them into some suitable box (which you could hide away in normal use), and you're done.
Use reasonably meaty speaker cable too - the current is starting to get serious here :).
Originally posted by Mark Ward
micb3rd - You're from Bedford and you also work for SVS?

From what you say I could power 2 Passive SVS subs from one of my H/K amps with good results?

They are bridgeable but they will offer 4 Ohms: 170 Watts @ <0.08% THD per channel if used separately.

This sounds like good news to me. One thing.. What does the Samson amp SVS sell offer over a regular 2 Channel power amp?

Things are looking up.


Yes I do live in bedford, but I do not work for SV Subwoofers.

In a small room 170 watts RMS would be fine for approacing high SPL levels, but you have a rarther big room so I agree with Ramius that more power is required.

I think the rough RMS value of the 20-39 is arround 350 RMS but more will not hert it at all.
Stoopid me! Sorry Micb3rd, I didn't spot this bit
It is on their site......Here is a small section for you...
When I read the "We" bits I read it as coming from you.

Originally posted by Mark Ward
Stoopid me! Sorry Micb3rd, I didn't spot this bit
:rolleyes: When I read the "We" bits I read it as coming from you.


No worries, good luck with your SV Subwoofers they should sound great.
Hi Mark,all.

Since you already have the HK amp channels on hand...I would recommend trying the SVs out with them to start. Going from 170w to 500w will give you another 4-5dB of headroom(max clean output capabilities)...but you may never feel the need for more bass once you have the twin passives dialed in with 170w each. And while you are enjoying that setup you could always keep your eye out for a great deal locally for a 400-500w per channel stereo amp.

All the SVs represent a 4ohm load to the amp.
You can flip the phase 0/180 on the passive subwoofer by just reversing the polarity.
I would go with the least expensive 12-16g wire you can find. 16g is fine for runs up to 15ft or so...14g up to 25ft...anything 25-40ft, stick with good 12g.

how about a couple of bridged Nad C270 power amp (about 600wpc @4 ohms)
That would do the trick :)

The Samson-1000 in $459 from SVS to even with duty & VAT it's a very good buy for powering subs.

Right now they've some B-Grade stock of these @ $365 which is amazing value. I don't know the duty rate for power amps but if it's 4% then the B-Grade stock Samsons will come in at around £285 + substantial shipping.

[EDIT]Actually there would be duty & VAT on the shipping costs too. Don't you just love UK C&E?

I've mailed Ron at SVS to cjeck on the International Warrantee status of these amps and the lead time on the Subs.

I use a couple of Bridged Rotel 981's for my Subs.
The more power I put into them subs the better they sound.

Just looked at the SVS website, those are cool looking subs :cool:
not too big either. I like.

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