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Speaker question

Tube Radio

Tube Radio

Super Member
I have a Scott LK-72A amp with 4 ohm and 8 ohm taps on the output transformers. My DIY home speakers consist of one 4 ohm subwoofer per channel along with a 8 ohm midbass 8 ohm midrange and 8 ohm super tweeter per channel. All is on a crossover I built myself. What I was wondering is would it be possible to run the 8 ohm sections of the crossover on the 8 ohm tap while running the 4 ohm section on the 4 ohm tap?
 
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Most amps with output trannies will take umbrage to having different taps used for multiple speakers. In other words, you'll let the magic smoke out. Unless the woofer has the same sensitivity as the the midrange and tweeter, the levels won't match up. This is where using a seperate amp and electronic crossover can be beneficial. I know what you mean by *too much bass* with many active subs...but that has less to do with the amps driving them and more to do with the enclosure's tuning and room placement.
 
What I did was to leave the choke connected in series with the sub while leaving the added capacitor to my current crossover connected in series with the midbass driver. I had figured the 8 ohm tap would only see a load above the crossover frequency and the 4 ohm tap would see a load only for the frequencies below the crossover frequency. I may try active subs once I get the money to do so. Right now I just need something to get by with until I can get the subs. My current subs are not as sensitive as my 12" Magnavox speakers I'm using as midbass drivers are. I plan on replacing the Magnavox speakers with these http://www.parts-express.com/pe/showdetl.cfm?Partnumber=290-510 per the recommendation of GordonW. I'm only using these subs, until I have the money for these http://www.crutchfield.com/S-ZAxivG...h=12"+subwoofer&tp=111&tab=features_and_specs
 
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Sounds like your approach is like running 2 speakers on the same channel in parallel...so you mght expect total impedance to drop below 4 ohms....
but what the heck...try it...you won't explode the unit....
 
I've tried it and see no problems yet. Not sure if the impedance would drop below 4 ohms as the 8 ohm tap due to the crossover on the speaker connected to it only sees load above 159 Hz and the 4 ohm tap only sees load below 159 Hz due to the choke in series with the subwoofer. Not sure how correct this is though. I'll post the crossover with how I have it connected to the amp to further clarify what I mean.
 

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So far it works nicely, but I will keep an eye on it though. Only temporary until I find another way to power the subs. I'm debating on whether I should connect a solid state amp to the speaker terminals on the Scott or use a separate preamp and use a two way crossover. I would rather not use a separate preamp as that adds components to the signal chain which I do not want except as a last resort.
 
Good luck, you have created a nice formula to destroy a nice amp and potentially a home/apartment, lives, etc. Keep up the good work.
 
dnewma04 said:
Good luck, you have created a nice formula to destroy a nice amp and potentially a home/apartment, lives, etc. Keep up the good work.
Click to expand...
I am sitting within 2 feet of the amp and the amp is not in its cabinet, because the new output tubes are a little too tall. If a tube plate does start to glow red (I doubt it ever will) I will see it and kill the power immediately. Also given the amp is fused if anything happens the fuse should blow. I already know the risks. I am happy to report that everything seems to work properly with nothing getting hotter than usual. I've had it running like this since about 3:15 PM today. If I had any doubt whether this could cause damage or not I would have not done it.
 
dnewma04 said:
Good luck, you have created a nice formula to destroy a nice amp and potentially a home/apartment, lives, etc. Keep up the good work.
Click to expand...

Actually, if Mr. Radio has connected everything up as he states (4 ohm LF on 4 ohm tap thru LP inductor and 8 ohm HF on 8 ohm tap thru HP cap) for all practical purposes, the amp will see the intended load at the anodes ov the finals. This of course assumes the inductor and cap are of complementary values. Even if the values are a bit uncomplementary, no harm should come to the amp.

Generally speaking, tube amps can handle lower than intended impedance w/o issue, esp if not pounded with power. Infinite load impedances while being driven are what tube amps with output transformers fear most.
 
I have a Magnavox SEP 6BQ5 stereo amp that with its original output transformers would break into oscillation with no load connected and volume control turned to minium. I measured that voltage once at 70 volts AC.
 
Listening to Henry mancini's album In The Pink and it sounds like the flute played by james Galway is right in front of me
 
Tube Radio said:
Listening to Henry mancini's album In The Pink and it sounds like the flute played by james Galway is right in front of me
Click to expand...

I agree with Steve. I don't see how this could damage anything.

The only issue is the voltage going to the mids/tweeters might be a tad higher than the voltage going to the woofers. You'll just have to use your l-pads to bring everything back into balance.

Actually, by having a good impedance match for both speaker halves you should get less power loss in the output trannies and consequently they just might run a tad cooler.

Regards,
Jerry
 
I do plan on trying a separate amp for the subs. Not sure if I want to use a separate preamp with two way crossover or if I want to connect the amp to the speaker output terminals of the Scott. I do have a Sony TAE5450 preamp I can use if a preamp is the best way to go, although I need to fix the volume control as one section is not workijng properly. Given the volume knob uses a set screw I may replace the Alps stepped attenuator with a good quality Alps dual gang variable resistor.
 
Ok I have an update. I am going active with the subs. I am driving them with a Kenwood KA-5002 amp I picked up Saturday for $125.00 cash. After repairs due to someone replacing an output transistor with an unsuitable substitute (replaced all four audio output transistors) the amp works nicely. I tried the amp running off my Scott LK-72A 4 ohm speaker terminals since my Sony TAE5450 preamp needs a volume control and it sounded decent. I then remember that I have a Fisher 202 Futura Series receiver (has a bad driver and power amp section that I could not fix) that I made into a preamp by taking the audio inputs to the driver board and routing them to the AUX 2 input jacks removing the original AUX 2 input wiring. I fed the Fisher into the Kenwood and fed the Kenwood's record output jacks into the tape in jacks on the Scott. This system sounds sweet, but will sound better once I get the Sony preamp fixed. I also found out that my subs were incorrectly wired for reverse polarity which may explain why they didn't sound good originally.
 
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