Gainclone and speaker protection

sts9fan · « **on:** 10 Sep 2007, 01:07 pm »

I have a question regarding the building of amps and potential speaker damage. I have finally built my gainclone from peter Daniels premium LM3875 kit at www.audiosector.com. I have not added anything other then a transformer, switch and fuse. Does this amp have the potential to damage my speakers? I have a pair of DIY fostex that I am using it with and it sounds great but I am hesitant to try it with my Druids. $32 drivers are worth the risk.
I guess my question is how do I guard against damage?

Thanks
Kris

danubs · « **Reply #1 on:** 10 Sep 2007, 03:21 pm »

On the gainclones, biggest problem is from the outset when some wrongly wired - end up sending near rail voltage to output. Or later if something shorted. Also, make sure sources or pres present no dc as the 3875 kits tend to not use a certain cap to restrict DC gain. Thus have a couple hundred millivolts can become few volts (voltage gain of 25 or so). Some will put input caps. I don't as i know my sources/pre's have no dc offset or have caps.

I think there was a kit from velleman for dc protect. Someday i need to dig out some no frills dc detect/protect myself.

Steve Eddy · « **Reply #2 on:** 10 Sep 2007, 04:24 pm »

If you want input DC protection, a CineMag CMLI-15/15B input transformer would take care of that and give other benefits as well (such as ground isolation). For output protection, short of a DC sensing circuit, you're pretty much limited to using a large value, non-polar electrolytic. With bi-wireable speakers (which the Zus don't seem to be) you can run the cap coupled output to the low frequency input and go straight into the high frequency input where the series capacitors in the crossover will provide DC protection for those drivers.

se

sts9fan · « **Reply #3 on:** 10 Sep 2007, 05:17 pm »

could I use a pair of these to prevent DC?

http://www.designnotes.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=M4701B&Category_Code=hiehp

Steve Eddy · « **Reply #4 on:** 10 Sep 2007, 05:32 pm »

Don't see why not.

se

sts9fan · « **Reply #5 on:** 10 Sep 2007, 07:42 pm »

Other then that how would I use non-polar caps? What size would be suitable?

Steve Eddy · « **Reply #6 on:** 10 Sep 2007, 08:51 pm »

Quote from: sts9fan on 10 Sep 2007, 07:42 pm

Other then that how would I use non-polar caps? What size would be suitable?

2,200uF to 4,700uF should do you. You can also abut two polarized caps together to make a non-polar cap, but the total capacitance will be half that of each individual capacitor. So for example if you used two 4,700uF caps, the total capacitance will be 2,350uF.

What's the power rating for your gainclone? That'll determine what the voltage rating for the caps must be.

se

sts9fan · « **Reply #7 on:** 11 Sep 2007, 12:16 pm »

Quote

What's the power rating for your gainclone?

Based on the data sheet, my transformer and 12 ohm speakers it would be close to 35W. I guess that would be close to 50-60 into 4ohms

So I just attach the cap between the speaker outputs?