Adding a pre out before the chip

[meepers]

Here is what I am trying to accomplish. I am trying to add a pre out to a radio unit that does not have a low level out. The unit currently is running two of the worst spec amps I have ever seen TDA1560q's that put out only around 10w at 8ohm loads. As this is a motorcycle radio, you can imagine at highway speeds this is unacceptable. I do not wish to cut up the factory harness on this bike nor use line level converters into a second amp. I will be running a normal set of components that are 4 ohm as well.

I want to run a short set of preouts to a better spec chip amp mounted directly above. I currently have a few from other car radio's and also a very nice set of 50x4 but that would require me to build a switching psu. Swapping the radio out is not an option right now as I would loose the CB and intercom feature which is nice on long rides.

What I want to know is what I would need to interface a set of RCA's just prior to the chip amp for the front channel as the output of this would not be connected. I dont know what is needed or normally used to isolate the additional output from feedback or any other issues that would arise. The board is fairly easy to access on the unit, and I am not opposed to removing the existing amp to the front completely, but whatever changes I make, I want to be able to undo them to the headunit. I also have a few el cheapo car radios that I can practice on before actually doing the work on this unit.

THe amp I think I am going to replace this one with is a TA8251 which has nearly double the power and its 4 channel. This amp would allow me to Bi amp the speakers for better overall output.

I also have a set of TDA8594's I eventually want to use, but they require dual rails, and the current infinity amp they are in use the I2C interface. the spec sheet shows promise on this as the i2c is bypassable, but the power supply is still to advanced for me.

here is the PDF link to the tda1560

http://www.datasheetcatalog.org/data...TDA1560Q_4.pdf

Any direction on this would be much appreciated as I really dont want to toast this $700 radio. I can photo anything thats needed as well. Its nice to finally after all this time really have a reason to work on this and build an amp as well.

[pacificblue]

The datasheet says it puts out 40 W into 8 Ohm loads (class H). That means you have to assume around 18 V maximum output. If it is given as 10 W only, then it must be in class B mode. That would be around 9 V maximum output.

What you need is a simple voltage divider that could even fit into an RCA connector, if you are lucky. Lucky meaning you find small resistors, a connector with a big housing and you have calm hands for soldering and fitting everything together.

Now you need to find out the gain of your new amp. I couldn't find the IC you mentioned. Let us assume 20 times gain and a maximum output of 25 W into 8 Ohm. That would be around 14 V maximum. 14/20=0,7, so you want a voltage divider with a ratio of 9/0,7. Try an 8,2 kOhm resistor in series and a 680 Ohm resistor in parallel to the new amp's input.

Before doing that, you should consider to buy more efficient speakers instead. You need 10 times the power to achieve the impression of doubling the volume. And then your speakers must be able to deal with that power not only electrically but also mechanically. With motorcycles it is usually the decision of listening to music or driving at high speeds, because the speakers you need to cope with the surrounding noise would have to be really big. Or, but that is of course not allowed , use quality headphones below your helmet.

[AndrewT]

what's wrong with using earphones inside your helmet?
Then the amp only has to drive tens of mW into high impedance.

[meepers]

For local around town riding, I only wear a 1/2 helmet. But for long trips I generally do use the headphones with my full face.

The new speakers are 4 ohm speakers. I currently have them running off a Pioneer head unit. All the amps I am looking at using are all 4 ohm rated as well.

http://www.radio-portal.ru/book/%D1%...B/TA8251AH.PDF

Thats the link for the chip I am thinking of using in a separate enclosure. But even for the time being, I have a few small 25x2 car amps that I can use until I get my unit built, and I would like the option to add later if I need to.

Pacificblue, I reread your post and I think maybe there was a misunderstanding. What I want to do is add the line out BEFORE the chip amp used in this enclosure, or even remove the amp completely that came with the unit. The factory setup is all 8ohm, and the units amp is very weak. I figured by nearly doubling the power, and using a better amp, I would not only gain the ability to use auto 4ohm speakers and crossovers, but also get a little more volume and clarity for round town romps.

When I get home today I will draw up a diagram and maybe that will clarify.

Thanks for the quick reply's guys!

[pacificblue]

Originally posted by meepers
[B]Pacificblue, I reread your post and I think maybe there was a misunderstanding. What I want to do is add the line out BEFORE the chip amp used in this enclosure, or even remove the amp completely that came with the unit.

Right, I got that wrong. In that case you should look for the blocking capacitors (Ck in the datasheet) and take them out. Their input pins on the PCB are the easiest place to connect your RCAs to.
Removing the IC will make sure that it doesn't consume power. If you dont want to do that, put a jumper across the input terminals.

Originally posted by meepers
I figured by nearly doubling the power, and using a better amp, I would not only gain the ability to use auto 4ohm speakers and crossovers, but also get a little more volume and clarity for round town romps.

Doubling the power means you gain 3 dB, which is not a big difference. You should rather expect improvements to come from a better frequency response in the lower regions with a different IC and from more efficient speakers.

[meepers]

Thanks! I will do some poking around on it tonight for those caps and the best place to connect the RCA's. I have an old factory car unit that I think I will test this out on first to make sure I get the results I am looking for.

I will snap some pics when I am working on it and let you know how it goes.

Is there anything that I need to worry about adding for voltage or signal going the other way?

Thanks again.