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AUDIO FORUMS >> D.I.Y. >> SE34.1
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Message started by 4krow on 07/20/10 at 15:38:02

Title: SE34.1
Post by 4krow on 07/20/10 at 15:38:02

I plan to use my SE34.1 amp. as a mid/treb. amp. in a biamp. setup. All I would have to do is install a cap that will limit the amp to 70-80hz and above. I'm not sure of the input impedance of this amp(I suppose I could measure it), and wonder the size of cap needed to accomplish this task. Anyone got an idea about this?
Another possibility would be to mod this amp with a 'pre out' and 'main in' set of jacks since I am using a Velodyne SMS-1 as a crossover.

Title: Re: SE34.1
Post by dank on 07/21/10 at 18:00:14

Typically, a subwoofer plate amp will offer a cap to filter the lows out of the signal that goes to your main speakers. In this amp
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-804
they use a 220uf capacitor, 100v on each channel and claim its a 6db/octave 125 hz hi pass filter. You can use this calculator
http://www.diyaudioandvideo.com/Calculator/XOver/
to adjust...according to the calculator, a 5.8 ohm tweeter at 125hz results in 220 uf, so I guess they are using an 8 ohm speaker with a DCR of 5.8.

This (non polarized) capacitor would go on the output of the amp in series with the speaker. If you wanted to put the cap at the amp input, I'm not sure how you would do that. You would probably need some sort of RC network, as I don't think you could successfully use the input impedance of the amp as the R like you can when you are dealing with the amp output and a low resistance speaker.

Dan

Title: Re: SE34.1
Post by 4krow on 07/22/10 at 19:06:38

thank you for the calculator site. It makes it fast and easy. As I expected, the value of the cap in this case is about .033uf since the impedance of the input of the amp is 47,000 ohms. The cap can be applied after the selector switch in the amp and before the input tube. The velodyne has the same cap in it, but you need to have a pre out & main in when using an integrated for this to work. Considering my intended purpose here, I can eliminate using more patch cords, keeping things simple. Off to soldering I go..... Greg [smiley=10.gif]

Title: Re: SE34.1
Post by dank on 07/23/10 at 00:08:13

I would be very leery of that 47,000 ohm input impedance. I don't think its constant over frequency, and I don't think its very accurate for your particular unit but rather an average value meant to cover all units and might very well be a guaranteed minimum.

I suspect you would get more accurate results if you added a R to ground in parallel with the Zin of the amp and about 1/3 of the Zin value. So in your case, 15k or so. This would make your new Zin of the amp in parallel with R a little over 11k and the resulting cap would be around .16 uf from the calculator. Any source worth its salt ought to be able to drive 10k, and the R you install will reduce your dependence on Zin of the amp being exactly 47,000 ohms.

Dan

Title: Re: SE34.1
Post by 4krow on 07/23/10 at 01:31:28

Dank,
As it turns out, I measured the zin at 101,000 ohms. Of course, as you said that is at a specific frequency. But, after plugging in the necc. data, the calculator came up with .022uf for a 80hz cutoff....Not wanting to bust open the amp until I find a suitable cutoff freq. I reintered the info as though I was going use it at the output of the amp. This is an easier way for me to determine where I want to set the cutoff frq, then I will consider how I might implement it in the input of this amp. (the speakers that I am using are single driver, so using a single cap should be fine). I guess time will tell....Greg

Title: Re: SE34.1
Post by 4krow on 07/29/10 at 03:53:33

Well, I know that it might be lame to answer my own posts, and yet, this may be worth mentioning. The cce mod that I did on my 34.1 has had a surprise. Yah, I liked the sound of it when I used the 'bass switch' in the forward position, and figured it would stay that way. Then today, the caps came in for the speakers I'm using, making them receive approx 80 hz and above from the 34.1. Can't really explain it but the sound seemed to lack treble, and I thought maybe I have miscalculated the cap value. But, just cuz, I switched back to the 'british sound', and man,,,,I am there. I mean if I change one more thing, then I know that I am crazy! This exactly the sound I thought would never come back into my system.So, if I talk about v caps or any more messing around, stop me...Just stop me....Greg