Ashdown ABM - modding pre shape circuit?

[SwamiRob]

Hi there, was just wondering if anyone's messed with the preshape on an Ashdown preamp before? I like the extended highs & lows with it on, but the massive 400hz cut ruins it, and there's no way of boosting the mids in a way that really gets the same time back when I've tried. Running flat with a massive treble boost, a bit of a bass boost, cutting that horrid middle pot 600ish frequency, then doing a cut to the low 160ish and 2k sliders seems to come out quite nicely; still doesn't have the depth & sheen of the frequencies the pre shape gives though.

Can anyone see/does anyone know if there's an easy bypass to the mid cut of this circuit? It looks a little more complicated then I was expecting (not that I can really read them...) when I looked at the circuit diagram. If it's a matter of disconnecting a couple of resistors or caps I'm definitely up for trying it.

Cheers for any help!

 

[agedhorse]

Clear as mud.

How about just turning the shape circuit off? There's a switch for that.

 

[SwamiRob]

Wow, not quite sure how that diagram ended up in such horrendously low quality... I'll try sort that but can't seem to find a way of editing the OP on the mobile site (and the typo in it - "same tone back" not time!). EDIT - the one below should be a little more coherent, still can't see how to edit the OP though.

I've played without the preshape most of the time over the last 7 years or so I've had it, just feels like it's really missing something for the sound I'm after. The bottom end can be filled out plenty with some of the signal splitting I'm doing before the amp, but the top end is nowhere near as bright.

 

[Wasnex]

Wow, not quite sure how that diagram ended up in such horrendously low quality... I'll try sort that but can't seem to find a way of editing the OP on the mobile site (and the typo in it - "same tone back" not time!). EDIT - the one below should be a little more coherent, still can't see how to edit the OP though.

I've played without the preshape most of the time over the last 7 years or so I've had it, just feels like it's really missing something for the sound I'm after. The bottom end can be filled out plenty with some of the signal splitting I'm doing before the amp, but the top end is nowhere near as bright.

I don't know if this is accurate for the specific series you have.

This is not subtle, so I am not surprised that you don't find it usable. IMHO the best solution is to select flat, and then adjust the tone controls to taste.

I don't have a deep enough background to really understand how the circuit works. But I believe the "Shape" signal path is shown in orange.

The components in the blue rectangle are responsible for the tone shaping.

When the button is set to Flat, the signal path is straight through TR15. AFAIK TR14 and TR15 function as gates, and only one passes signal at a time.

Honestly, I think the best solution is just don't use the "Shape" preset. Nothing says you have to like every setting the camp is capable of. For example, I don't really care for the presets on my Mesa M9. I select the flat preset and use the amps tone controls.

Tone control specs:

Should be penty of options. IMHO, if you can't get the sound you want by adjusting the tone controls, it's probably time for a different amp.

 

[SwamiRob]

Thanks for the detailed response. Yeah it's quite a brutal change isn't it! I'm not sure if it's specifically for my model, but it's certainly around that era, and I don't believe that pre has changed a great deal over the years anyway to my knowledge.

Don't really have the money for a change at the moment unfortunately though, I'd definitely like to try something different although I do quite like it it's not quite what I'm after. It'll certainly do for now and it's very versatile. 1 useful feature the Ashdown also has is having a line in, which is great as I'm splitting into a slightly grindy clean tone driving the valve preamp a little, then having a distortion channel through a rat clone into a VT Sansamp which I stick in through the line in. All works pretty well, and with a nice Barefaced 6x10 it's a pretty decent sound. If it's a case of just clipping a couple of easily reattached components I'd quite like to try getting that mid notch out of that circuit, as if I could just make the thing a little brighter I'd be pretty happy with what I've already got to be honest! It's something I wouldn't mind trying as a pretty much free solution as I can't really look into anything else for now!

Would I be correct in saying that the mid cut is first up in that circuit, before the IC presumably drives the boost for the bass & treble? So if I stop R30 going to ground that won't happen? Or with it been an active circuit would it not be something that straightforward? My electronics knowledge is very limited!

 

[Rogulus]

My only complaint about the ABM range is that shape imparts such a radical mid scoop that it's unusable, great bedroom switch.

I'd flick it on for maybe dub/Reggae tunes but don't play in any projects that require that sound.

 

[SwamiRob]

Yeah it's a bit naff, I like the extended top & bottom, just not taking so much midrange in a frequency area I quite like away!

 

[agedhorse]

No, R30 has little (nothing) to do with the mids.

It's not a simple DIY change. Leave it alone or you might find that you then have other other more serious issues.

 

[Sparky Mark]

Would something like this help?

 

[Wasnex]

Thanks for the detailed response. Yeah it's quite a brutal change isn't it! I'm not sure if it's specifically for my model, but it's certainly around that era, and I don't believe that pre has changed a great deal over the years anyway to my knowledge.

Don't really have the money for a change at the moment unfortunately though, I'd definitely like to try something different although I do quite like it it's not quite what I'm after. It'll certainly do for now and it's very versatile. 1 useful feature the Ashdown also has is having a line in, which is great as I'm splitting into a slightly grindy clean tone driving the valve preamp a little, then having a distortion channel through a rat clone into a VT Sansamp which I stick in through the line in. All works pretty well, and with a nice Barefaced 6x10 it's a pretty decent sound. If it's a case of just clipping a couple of easily reattached components I'd quite like to try getting that mid notch out of that circuit, as if I could just make the thing a little brighter I'd be pretty happy with what I've already got to be honest! It's something I wouldn't mind trying as a pretty much free solution as I can't really look into anything else for now!

Would I be correct in saying that the mid cut is first up in that circuit, before the IC presumably drives the boost for the bass & treble? So if I stop R30 going to ground that won't happen? Or with it been an active circuit would it not be something that straightforward? My electronics knowledge is very limited!

My guess is R30 sets the level of signal attenuation at that point of the circuit, and also determines the frequency where the lows start to roll off.

The way I look at it, you currently have a functioning amp with a preset you don't like. IMHO simply don't use the preset. Setting the button to flat totally bypasses the Shape circuit.

If you start clipping components, it's likely to make matters worse, so you still won't like the preset. Also you may damage the amp in the process, which will force the need to either replace or repair it.

I don't think the solution will be clipping components. Usable results will likely require changing the values of various components.

IMHO this is not a great platform for experimenting. If you want to mod the circuit, the first thing would be to understand exactly how it works, so you can make an informed decision on how to proceed. You would probably want to model the circuit in a program like SPICE. I have no experience with SPICE, but maybe it would allow you to experiment and figure out what changes are likely to produce the desired results.

 

[Passinwind]

I don't think the solution will be clipping components. Usable results will likely require changing the values of various components.

IMHO this is not a great platform for experimenting. If you want to mod the circuit, the first thing would be to understand exactly how it works, so you can make an informed decision on how to proceed. You would probably want to model the circuit in a program like SPICE. I have no experience with SPICE, but maybe it would allow you to experiment and figure out what changes are likely to produce the desired results.

Apologies, had to delete my last reply due to a misread of the schematic. The pesky one hour editing window still bugs the crap out of me, frankly.

Here's my revised LTspice model of the blue outlined part, noting that the actual composite curve factoring in other stages can and will will be much different. This is a simple passive notch filter and there is presumably makeup gain elsewhere. Offset it upwards by half then roll off highs and lows a bit and and you're getting in the ballpark of the numbers you posted. So yeah, DIY modding is not all that straighforward here.

 

[burgerdj]

Add an external eq to add back the lost frequencies. I would not modify the amp at all.

 

[SwamiRob]

Thanks again for the responses (especially Passinwind for simulating the components! Ta for that, that's much much more extreme then I was expecting...) I wasn't aware that the circuit was quite so complicated to mod, which is the initial reason I asked. I figured the worst I'd do is break an EQ I wasn't using anyway.

I just want the thing to be a little brighter, and there's a massive gap in tone adjustment between that 2kish slider (which I hate the sound of) and the 6-7kish shelving which I do, however is obviously very very toppy and I'm probably only hearing much of it with the cab I'm using because I'm cranking it pretty much full, and I assume I'm getting something a little lower than that in reality. I've already got alot going on before the amp so would rather avoid more pedals, but I'll try do something in that bit of the signal chain though. I've got an FEA dual band comp which bypasses over 5kish then mixes it back in, I'll try to turn down the top band compression post gain so there's a bit more top end in comparison, or mess around with signal splitter as that's maybe got something in the top end section I can mess with without causing any harsh clipping.

 

[lowmid]

I'm actually considering this, bear in mind, I'm buying a cheap ABM 500 head to mess with it a little. I'm thinking of removing TR14 and TR15 and putting a blend pot between flat an shape. makes sense since all paths are buffered. something like a B50K pot could work. I'm using this in my own design to remove some mids before distortion, and it really helps some basses to remove some pokey mids .

I'll let you know how this goes!