Newbie question on protection circuit

[senderj]

My pre-amp has DC at the output everytime I switch it on or off. When switch on it reaches 10VDC at 6 second and then drop to 0V. When switch off it reaches 20+VDC at 12 second and then drop to 0V. BEfore I can fix this I need a time delay in it.

I found this one in the Net. The relay part is not very clear. But I am planning to do is, in parallel to the signal out, connect the relay so that, when power off, the relay connect the signal out to ground. When power on, after th time delay, disconnect from ground. But I have questions that need your help:

1. If normally connected to ground, does it means 0 load to the pre-amp? Will this damage it?

2. The circuit has a table below it for selecting the delay time. But I don't really know how to calculate it. Can somebody tell me how?

Thanks.

[rpapps]

Can you post a schematic of the pre-amp as I think the problem lies in there. You need to treat the disease, not the symptom.
Cheers
Rob

[richie00boy]

I agree with Rob, but the circuit looks OK. It might damage the pre-amp but can't really say without seeing it's schematic.

Do you actually have a problem with speaker cones moving a lot at turn on/off then? Because really you should turn power amps on last and off first, then you avoid these problems.

[AndrewT]

Hi,
I agree with the other replies, try to find the problem or confirm that the preamp is behaving normally.

The circuit posted will indeed delay the relay to switch on.
I think it will also hold the relay on for a short delay while the capacitors discharge. I don't know how long the off delay will be.

You need a delay on and instant off.

If you go ahead with the circuit then a slight modification is advised. Add an electrolytic capacitor from diode D10 anode to Transistor N2 emitter. 25V 33uF should do. It will let you run the relay at a lower current keeping it cooler and also allows it to switch off more quickly. This is achieved by increasing R2 to reduce the continuous voltage applied to the relay when in the on state.

[peranders]

A working delay circuit like yours can be found here.

[senderj]

Thank you for all the replies. I do agree that it's the DC that need to be fixed. But I couldn't. My pre is a typical GG. I have posted question to Transcendent's message board but the replies couldn't help me fix the problem. There was a post back in 2004 on the same issue. Bruce's reply on the cause of the DC was that the coupling cap discharging through the tubes. And no fix was provided.

The DC issue only happens when switching on or off my pre. Other than this, it works normally and sounds great. Before I can fix the DC issue, I can only use a time delay to mute the output.

Rob, the schematic is the same as the original GG with CCS. Not sure if I can post it here.

richie00boy, the DC issue costed me a pair of bass drivers.

AndrewT, thanks for the suggestion. With the current value of the components, the OFF delay is 3 seconds and very consistant, so enough for my case. However, the ON delay varies from 85 to 98 seconds (instead of 50 seconds as in the schematic table). So I need advise from this forum on how to calculate the time so that I can adjust it. Could you help me on this?

peranders, your suggested circuit is on the HT side. I prefer a mute at the output which is cheaper and smaller footprint. THanks anyway.

[rpapps]

Assuming you purchased it from Transcendent, if i'd spent USD$500 on something that was causing me grief I'd certainly be giving the supplier an earfull to get it resolved rather than having to outlay more money and time to work around it.
I did some Web searching and what puzzles me is the number of favourable comments from other purchasers. If your situation is unique then I think you need to resolve it with your supplier.
On the other hand, if you've used / modified their schematic to make your own then we need to see it to offer useful advice.
Cheers
Rob

[AndrewT]

Hi,
have another look at Peranders' link.
It shows a slightly different topology and could give you some hints on modifying your current delay circuit.
I would certainly recommend you alter your circuit to achieve instant mute when going to "off"

What output impedance does your GG use/have?
The mute should go after this to prevent a dead short on the output to ground.

[senderj]

Rob,

I didn't purchase from Transcendent, but build it myself with the same schematic. Here is the link that lead to the subject
http://www.websitetoolbox.com/tool/p...post?id=134954
which means other GG owner also has the same problem. But I may be "unique" since it will cause problem only when link to SS amp that without any output protection.

AndrewT,

The output imp is 200 ohms. But as said, the off delay is fine. I wanted to master the on delay time. I don't know what value of the C and R produce a 50 second delay.

[AndrewT]

Hi,
R1 and C2 combined create the RC time constant. The time delay is directly proportional to the time constant.

Increasing C2 increases the delay. Increasing R1 also increases the delay.

You require a shorter delay, then either reduce the cap C2 or reduce the resistor R1 or do both.

Since the cap is the more expensive and removing it from the PCB may damage it, it makes more sense to change the resistor. You could try a trial and error method or calculate the new time constant.

The old RC gave 90 seconds, you want 50 seconds.
The new RC should be 50/90 times the old RC. This won't give an exact answer but choose a new reistor close to give the new RC you need.