P7 Pre amp Trouble

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Hello, I'm putting together a amp using some cheap L20 amplifier boards from E-bay. The amp sounded great, but didn't have enough gain so I picked up a pre-amp for it that was recommended to use with the L20 amplifier and wired it up today. When I turned it on the bridge rectifier burnt up on my speaker protector which is hooked up to the same transformer output. The speaker protector gets 15 Volts AC single and the pre amplifier gets 15-0-15 volts AC and this comes from a tap on my main transformer.

I pulled the power from the speaker protector and tested the voltage and everything looks good, but there seems to be DC voltage running through the pre amp which could have been the cause for the damage to the speaker protector, but I thought the relays would have clicked off if there was an issue.

I measure 15-0-15 volts AC going into the preamp, but there is 18 volts DC coming out of the outputs and when I have a pot switch hooked up I can measure around 6 volts DC coming out of the input. This can't be right? Can any one shed some light on what is going wrong?

Here is a picture of the amp and pre amp. I have a lot of the wires pulled so I could just test the pre amp so I'm sorry if it's hard to see how it was all wired up.

Thanks for the help!
Dale P.


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I think we will need more pics of how exactly you have this wired up. Also, close up pictures of the preamp. Generally you will see a line level output which is waaay lower than what you say you are seeing at the output. Please take some pics of the wiring and some close ups of the preamp board. I am assuming the speaker protector is between the output of your amplifier and the speakers? I am just trying to imagine how exactly you have this wired up.

I can say that rectifier you have on the preamp can handle a lot and can supply up to 1A from what I read on the datasheet. I am suspicious of the wires sharing the same AC line so if we can see exactly what you did there we can likely track it down to something interesting happening with the power and not so much with the PCBs themselves. You will definitely have to replace the diodes on your protector, and it is really unknown if this took out a bunch of other stuff too..
I will try to post some better pictures, but it's hard as there is not much room in the case and most of the wires are routed under the boards.

The transformer in the case has two outputs. 45-0-45 which is hooked up to a large bridge rectifier, then two 10,000uf caps and then to the two L20 amp boards. The second taps are 15-0-15 and I have a 15-0 going to the speaker protector as it only needs positive 15 volts and ground and both 15-0-15 running to the pre-amp as it needs both the positive and negative 15 volts to work. What I did was solder wires from the transformer to each unit so there is a Y at the transformer splitting the taps for each board.

I think the problem came from the pre-amp as it's putting out 18 volts DC at the output which runs into the amplifiers inputs. (I didn't know this until I fried the speaker protector) The amp then boosted this into who knows what voltage and then put it through the speaker protector. The speaker protector has a 3 second delay and as soon as the delay went off the bridge rectifier started to smoke. I disconnected the pre-amp and tried it again, but it kept smoking so I'm guessing the damage was to severe to stop it from burning up completely.

I picked up a new bridge rectifier to night and will try to fix the speaker protector, but can't figure out what's wrong with the pre amp as it should not have 18 volts coming out of the output. To me it sounds like the 15 VAC coming in is being multiplied by 1.4 from the bridge and then being directly sent out to the output to the amp. What I need to know is why the pre-amp is doing this?

Dale P.

Thanks for all the info, I think I have a good picture in my head of how you have it setup.
I am having a few thoughts, first you definitely should not be getting anywhere close to 18V on the output, when you rectify the AC from 15VAC you will get a good little chunk less than 15VDC. In fact it looks like you have two voltage regulators on that board as well. the 78L12 and the 79L12 those are your regulators, the 78 being the positive and the 79 being the negative. If these are not in the right place you can get some pretty wacky behavior from that guy. Either way you should not be getting 18V regardless.
Everything else on your board looks like it is in the right place per the board labels.

I think the first thing we should do is try to power just this board and check those regulators, if you do not see 12 with reference to ground on both of them, something is horribly wrong with those regulators and we will need to look into that further.

I think you are taking the right steps.. first get that amp and speaker protector working as it did before.

Another question, did you try this kit out with your amp prior to relocating the potentiometer control?

Let me know about the regulators and if everything works alright after replacing the bridge rectifier on your speaker protector and we can go from there.

Thanks a lot for the help! This amp is being built by my self and my 15 year old nephew and I had him solder up everything hims self. (I of course helped him out and gave all of the joints a really close inspection before powering anything up) The amp has been working just fine for the past three weeks, but now we want more volume which is why the pre-amp is needed as there isn't enough gain from the L 20 boards.

My nephew put together the pre-amp at his house and gave it to me when it was done. I took a really good look at it before I tried it out and it looked fine to me, however I never thought to check the voltage regulators to see if they were installed correctly. I only looked to see if they were in the right direction. I just checked the board and YOUR RIGHT! He does have them swapped around! I knew what the difference was between the 7812 and 7912, but never thought to see if he put them in the right spot. I will swap them around and try it out. I'm sure this was the source of the trouble and I have to thank you for helping me out!

You also asked if I tried this kit out with your amp prior to relocating the potentiometer control, and the answer is NO. I told my nephew to not solder in the pot so I could easily hook up the front pots to the board.

Thanks again for the help, I will let you know how it works after I swap the parts around and replace the bridge rectifier in the speaker protector.

Dale P.
Dale, that is awesome news!

That is really great that you are helping your nephew get into electronics, and I would say that the board looks very good from the pics you have so good job newphew. Everyone makes mistakes like that when putting stuff together, I sure have and that I always my first thought when something doesn't go quite right... how would I have screwed this up??

Definitely let me know how it sounds when you are done with it, I am sure its going to be pretty great and exciting for your nephew to see something come to life.


One of the rectifiers came out in two pieces so we had to order one which will take a week to get. I really want to get this amp back in action so we can see if the pre-amp helps.

My Nephew built his own book shelf speakers from scratch which is when I thought it would be cool to build an amplifier to power them up. I was pretty impressed with his speakers as he used scrap lumber and ran it through a planner and made some nice looking speakers out of it.

The amp sounds really nice as is, but is only putting out around 60 watts and we want more. It's been suggested that the L20 boards could use a pre-amp so here we are. Hopefully we can get around 100 watts out of it in the end. The boards are rated for more, but my power supply won't do much over 100 watts per channel at 8ohms.
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