Simple resistor tweak for AR3a speakers used w/SS amps

[Guest lmrosenthal]

Hello everyone,

Since I've received so much help on this forum, I'd thought I'd share a very pleasant experience with you.

A couple of weeks ago, in the Vintage section of the Audio Asylum, there was a discussion about using Solid State (SS) amps that have a high damping factor (>100) with vintage speakers. One of the points made was that vintage speakers designed in the 50's thru the late '60's were voiced with tube amplification in mind.

It was mentioned that when AR was doing demos in the AR room during the 60's, they were using MAC's and Dynas for amplification.

One of the posters (Bold Eagle), suggested that a resistor (.5 to 1.5 ohm) wired in series with the speaker would present a speaker load to the amp more in keeping with a tube amp.

Since I have a pair of AR3's powered by a NAD C350 (60 watts rms/channel into 8 or 4 ohms) that has a high damping factor (~150), I asked Bold Eagle in a private email about applying the resisitor tweak in my system.

He responded as follows:

"Be happy to help you with this. I, too, have the NAD C350. It's a very well designed amp. But it has a very high Damping Factor (low source impedance). Tube amps have a low Damping Factor (high source impedance). So adding series resistance to the C350 gives it the impedance characteristic of a tube amp."

I purchased two .5 ohm power resistors, and wired them in series on one leg of each speaker cable. Total cost, $1.05. (I also purchased a pair of 1 ohm and 1.5 ohm resistors, but haven't tried them yet to see how increasing the resistance impacts the sound).

The results:

First, the biggest impact was that the whole mid-range and upper mid-range seemed to become much smoother. Next the treble seemed to have slightly better clarity. Finally, the bass seemed slightly more defined - a little less muddy if you will. Huge improvement? No, but noticable, and quite pleasing.

Am I hearing something better, or something thats just different? I don't know for sure yet. I spent some time switching the resistors in and out of the cables, and I like what I hear better with the resistors in the circuit than out of the circuit, so they're staying in.

I don't know if this tweak will work with more 'modern' AR speakers, but I would surmise this might work with the AR1 and AR2 series speakers as well.

If anyone else tries this, or has tried this, let me know what you think.

Regards,

Larry

[soundminded]

This might help by providing the amplifier with a higher load impedence rather than because it lowered the damping factor. Intuitively, lowering the damping factor doesn't seem like a good idea but KLH inserted a resistor in series with its woofers in some models to keep the damping factor no lower than 8. AR3 and AR3a had notoriously low impedences at some frequencies. We used this techinque in the mid sixties to protect a large Scott solid state receiver which blew up according to Scott because of the AR3's low impedence. This was their rationale for not repairing it under warranty. They said connecting it to AR3 was abusive. AR replied that they would sue Scott if they could get that in writing.

[Guest]

One way to keep from blowing an AR3a's mid-range and tweeters is to add 1 ohm resisitor in series with each of these drivers. Adding a 1 ohm resister in series with the woofer seems to be a strange idea. I drive my AR3a's with a Heathkit AR1500A (80 watts RMS/channel into 4 ohms). The receiver's power amplifier has a modification which keeps it from producing "square" waves when the volume is turned up very high. I've never managed to blow my tweeter or mid-range, but an AR1500A without the power amplifier modification could easily cause damage to these drivers.