[robjrock]

Well, I'm fairly new to amp building, but I just finished my first Cmoy and it sounds great! I'm really happy with it, but I'd love to add a simple bass boost function to it. After trawling for days through threads here and there, I've come up with a rough idea based on this schematic:

(Which basically takes the Bass Boost from the PPA)

Now to add it to the standard Cmoy board, I came up with this idea. And yes I'm sorry its so badly drawn...

Sorry, I'm really bad at reading schematics, so I was hoping the wonderful head-fi community might let me know if I'm on the right track.

Basically to explain my drawing, I would break the copper track between R4 and the far left / right jumper. From each point either side of the break, I would run a wire down (two wires for each channel) - each wire connecting to a 3-hole copper track below the normal Cmoy parts.

S7 would be the switch. When 'off' (or open) the BB would be on. Then by some sort of voodoo magic the C7 Capacitor (0.1UF I think) and the R7 (10k resistor?) would make the bass boost happen!

As I read it, the bass boost section must be part of the R4 part only, so it still runs 'parallel' with R5 (which in my Cmoy is just a jumper) - or would I be able to wire the bass boost section in simply between the L/R out and the L/R stereo jack?

Basically I'm hoping someone could let me know if I'm on the right track!? at this stage I'm only interested in having a switchable bass boost. And obviously I'd do the same for both L/R channels. Oh, is there any one to make it switchable by one switch instead of two? Many thanks

EDIT: Well i got bored and just had a go myself, as you can see in the photo. I used a 2.2 nF polyester cap and 100k 1/4 watt met-film resistor, no switch obviously, as a test. I thought I could detect a quite modest bass boost in the left channel (i only wired on channel) but it was pretty hard to tell - if it were there it was quite subtle! Any ideas? Different cap value?

 

[sanderx]

Quote:

Originally Posted by robjrock /img/forum/go_quote.gif Basically to explain my drawing, I would break the copper track between R4 and the far left / right jumper. From each point either side of the break, I would run a wire down (two wires for each channel) - each wire connecting to a 3-hole copper track below the normal Cmoy parts. S7 would be the switch. When 'off' (or open) the BB would be on. Then by some sort of voodoo magic the C7 Capacitor (0.1UF I think) and the R7 (10k resistor?) would make the bass boost happen! As I read it, the bass boost section must be part of the R4 part only, so it still runs 'parallel' with R5 (which in my Cmoy is just a jumper) - or would I be able to wire the bass boost section in simply between the L/R out and the L/R stereo jack?

The PPA bass-boost circuit is part of the feedback loop of the opamp & buffer combo, so if you want a similar circuit, you would need to wire it in series with R4, not replace R5 (which is outside the loop) with it. What the circuit does is essentially vary the gain depending on the frequency so that bass frequencies have higher gain than others.

As far as the switch, you have 3 options:

• No switch, the circuit always boosts bass with an optional pot controlling level
• Switchthat selects between cut-through and bass boost
• If you want gain control, a switch that selects both gain and bass boost at the same time ( a 4-way switch)

Broadly speaking PPAv2 (or cmoy) with this tweak is an active bass boost filter that also happens to have non-bass gain

You get a more powerful effect compared to applying the filter outside of the feedback loop.