Hi , Im working on a 2 channel guitar amp build and am trying to figure out a good way to do the channel switching .....

I first considered using some sort of fet switching but Ive heard it can alter the sound so now I"m thinking relays ......

First question .....

Are relays suitable for audio channel switching ??

If so will it cause any poping noise when switching ??
If so is there a way to minimize it ??

I was going to use one of the Power amp power rails to power the relays and was wondering if it will cause noise though the power supply when engaging the relays ??

Thanx

Nope , you don"t get something for nothing ....... If you want to use an 8 ohm speaker and have it as loud as when you use a 4 ohm speaker then get a more sensitive speaker ..... an 8 ohm speaker that is 3db more sensitive than your 4 ohm speaker will be just as loud as your 4 ohm and 6db more sensitive will be twice as loud as the 4 ohm .......

Cheers

Quote from: phatt on March 22, 2010, 08:49:37 AM

Quote from: Minion on March 21, 2010, 12:59:27 PM
I personally built my First circuits by designing and etching my own PCB"s which is a great thing to learn if you plan on building a lot of your own Custom projects but it is an art within it"s self ........ There are several free PCB design programs out there and tutorials on how to etch your own PCB"s then all you need is to know how to read a schematic and a Datasheet properly and you can start .....


Cheers

Yep I did that for a while ,,,till I realised it was painfully slow.
A bread board makes lite work of it all as you can quickly weed out all the hickups and worms.
THEN commit to PCB.
Phil.

I found the opposite to be true for me, I made far more mistakes when breadboarding than when PCB designing (I have a hard time adjusting from looking at the top of the board to the bottom of the board , the mirror image thing really screws with my Dislexia ) and I found I could design and etch a PCB in no time at all and end up with a better looking product .....

But I know a lot of poeple who swear by them so 2 each his own ....

Cheers

I personally built my First circuits by designing and etching my own PCB"s which is a great thing to learn if you plan on building a lot of your own Custom projects but it is an art within it"s self ........ There are several free PCB design programs out there and tutorials on how to etch your own PCB"s then all you need is to know how to read a schematic and a Datasheet properly and you can start .....


Cheers

You can also Bias Chip amps to Class A , Or so I hear .....

That amp uses a single Supply PSU so half of the DC Voltage will be on the speaker output.... Yes , It blocks DC , You could use a Different value but a Lower Value will Give you worse Frequency responce and a Bigger one will give you a better frequency responce so if you do use a different Value I suggest you use a Larger value .....

The schematic it"s self shows a 4 ohm load so you can run at least 4 ohm and higher loads and maybe lower as the output transistors are Paralell to the TDA2030 which should allow you run lower loads but don"t take my word for it.....

If it has a Line out then you can built a simple headphone amp circuit and listen that way or even buy one of those cheap behringer headphone amps and use that ....

Well you got a TDA7294 , there you go , you can get over 50w ....

There are a couple ways to Bridge chips , have one of the Chips set up as Inverting and the other as Non-inverting, I don"t like this method for the simple fact That it is Harder to gain match each channel with available Resistor values ......

The other method is either useing a Ballanced line driver (DRV134 , Or opamp/Transistor based) or to use an audio transformer ......

For a Bass amp I would go for an opamp based one as Ive had noise issues with DRV134 and it might be hard to get a audio transformer will low enough frequency responce for a Bass amp .....

I would also suggest doing some resistor matching to get the gain of each chip as close as possible .....

Cheers

Well the first thing you gotta learn even before building anything is how to read a schematic and learn what all the symbols in a schematic mean , then you got to learn the basic uses for the most common components like resistors, capacitors, diodes , transistors , IC"s ect ...... A simple kit for a Pedal is a good start especially one that has documentation that not only explains how to build it but also explains how the circuit works , PAia kits are good that way and they have so good designs which you can learn a lot from building but are a bit too expensive in my opinion .....
There are also some good cheap kit on e-bay , they are short on documentation but high on Value ......

One of the most Valuable things is to have a friend who knows his/her electronics who can help you understand things and to bounce ideas off of , which is one thing I never had ....

I personally spent over 6 months reading and doing research before attempting my first project , which by the way was a Simple Microphone preamp (Green Pre) which turned out to be a lot more difficult than I first thought it would be , I had to etch my own Circuit boards and source my own parts and i knew very little but the project was a success but not very pretty .... That was about 5 years ago and since then I have built Most of the Mic preamps in my Studio , Built my studio monitors , built a couple simple EQ"s and designed a simple active electronics circuit for bass and guitar which I have built and sold close to 50 of to friends and other musicians, at $35 a pop I have made a few bucks at it , Ive also built several guitar and bass amps from 15w up to 100w, Many sucked but every new one is better than the old one .......


Good luck

You might be better of Bridging 2 LM3886 chips instead of Bridging a LM4870 , Because the LM4870 is 2 LM3886 chips in the same package the Heat dissapation isn"t nearly as good so the thermal protection will kick in far earlier with a LM4780 than with 2 LM3886 chips spaced Appart .....

To create a Positive and negitive Audio signal for Bridging there are a couple options , the easiest but probably more expensive is to use an audio transformer , a 1:1 with a center tapped secondary would be good .....

the next option would to use a Ballanced line driver to create a ballanced signal from a Unballanced signal , there are Ballanced line driver chips like the DRV134 , of you can make one from a Dual opamp configureing them with one opamp at Unity gain Non-inverted and one at Unity gain Inverted ......

There are also some higher power chips out there , like the TDA7293/94 which can put out close to 100w into 8 ohms with a +/-42v supply or close to 200w into 16 ohms bridged ......

You can also rewire your cab to run at a lower ohms .....



Cheers

Personally for me Tone stacks suck , especially for bass , they only cut frequencies and dont boost and with a Bass it is nice to get more bass out of it than it normally produces .....

I would suggest you go for a active tone section , or even an active graphic EQ section (Parametric with adjustable Q would be much better but much more complicated) , it would be more versitile and allow for more tone shapeing .....


Cheers

I recently built a clean preamp for Bass or acoustic guitar , It used a simple dual opamp with a dual 100k pot to controll the gain (shared gain preamp) then it went into a BA3812 5 band EQ chip , so it had a 5 band eq and a clean preamp section with a single gain pot , I also added switchable clipping diode for a fuzz tone and a switchable Trebble boost ...... I run the whole thing on a single regulated 15v DC supply .....

very good sounding and dead quiet when you want it to be ....


Cheers

Well not knowing the hiwatt schematic I assume the preamp section has 2 or more Tubes ?? if you scale it back to one tube it really won"t be a hiwatt anymore and will probably have a much different sound than a real hiwatt ....

as for the output section the Hiwatt probably uses a Push pull output stage wich requires at least 2 tubes so to use just one tube you need a diffent output topology namely a single ended output stage , maybe something like the fender champ output stage .....

So maybe you can combine the hiwatt preamp with the Champ power amp ??