After Eighty - Small Ashdown Combo On Steroids

[stevie]

It's going to be a giant killer!😁

 

[Passinwind]

Hi John,

I've finally had occasion to break out one of the older revisions of the preamp in question, in the blue box to the right:

 

The power amp module is strangely big and heavy, and not particularly efficient...😎

[Chienmortbb]

17 hours ago, Passinwind said:

Hi John,

I've finally had occasion to break out one of the older revisions of the preamp in question, in the blue box to the right:

 

The power amp module is strangely big and heavy, and not particularly efficient...😎

It took me a while to get the joke. I was dazzled by the glass transistors behind the bass.

[Passinwind]

On 26/08/2020 at 00:35, Chienmortbb said:

It took me a while to get the joke. I was dazzled by the glass transistors behind the bass.

It's been quite a change of pace from all the battery powered circuits I've been doing for the last few years, 500VDC is no joke when it wants to bite you.

Haven't decided whether to do a B channel and use a set of the PW7 boards, but there's already a nice +/- 15V supply on the power amp board, and plenty of spare current capacity. The A channel will be mostly tube based in any case.

[Chienmortbb]

In my early years I tested TV studio monitors. one of the adjustments was near the line ouitput transformer. The LOPT (pronounced loptee) had a 1500V RF spike on it and, if you slipped, you had a nice little shock and a little hole in your finger. At least it was self quaterizing.

Now I must get on with the Combo.

 

Edited August 28, 2020 by Chienmortbb

[Chienmortbb]

I am shocked to realise that it is over 3 months since I posted. I did fit the port properly  after Stevie had checked the performance. Of course that ruined the finish. It needs some sanding around the port as well as a new coat of Tuffcab. The amp is being a bit of a B. Although the rear mounted speakon looks good, it does take up a lot of space, in addition to which, I cut the speaker cable from the power amp  a little short. So I am struggling to get the speakon and internal speaker connected. I do have a  faston speakon connector somewhere and that might be the answer.

The headphone amp is also a bit of a problem as there is no way of mechanically mounting it. If there was no headphone hole in the chassis I may have just left it off, although that severely limits its use as a practice amp.

My other dilemma is that the preamp power supply is a mess.  I used it to try a number of things and it just looks ugly. However it does work, so I am considering whether to replace it with some PCB power regulators (originally bought for a different project) or leave it as it is. There was a very small amount of hum (i really mean small) that was just audible with boty Volume controls at max, Changing the  power suppy regulators would also make it easier to rework the cable routing in an attempt to reduce the hum.

As it is raining here, I won't be finishing the cabinet any time soon so on with the amp section.

[Chienmortbb]

Sometimes I surprise myself how stupid I am. The original idea was that the D sized hole on the back would be the DI out and the one on the top would be for the  external cabinet output. 
 

The reason for this was that the original speaker output jack was on the top and the legend was still visible with the Speakon fitted. That skewed my thinking and it was this morning that thought to reverse that. 
 

So now the XLR is on the top and the extension AB output is on the back. 

The wiring  is complete although I still have to spray over the legend and find some way of printing gold on black. 
 

It should be dry tomorrow so  hope to Sam’s the port and get a cost of tuffcab on. The it’s joust a case of sorting the headphone circuit. 

 

 

 

[stewblack]

1 hour ago, Chienmortbb said:

The it’s joust a case of sorting the headphone circuit. 

Sounds like a knight to remember. 

[Si600]

10 hours ago, Chienmortbb said:

The wiring  is complete although I still have to spray over the legend and find some way of printing gold on black. 

Have you got any waterslide paper? You could play around with different colours of 'gold' on a sheet and try to see which one is best.

[Chienmortbb]

3 hours ago, Si600 said:

Have you got any waterslide paper? You could play around with different colours of 'gold' on a sheet and try to see which one is best.

I  have laser sheets but ly laser is momo only. I will buy some inkjet ones and see wht they turn out like.

[Chienmortbb]

After thinking about it for a while I have had a rethink. The headphone PCB is not easy to mount and I was.having second thoughts about using it. 

I went back to the blog post by NwAvGuy and looked at his measurements of the NJR4556 and it gave excellent results into 15 ohms. My headphones are rated at 32 ohms but my idea is to parallel the L and R earpieces from one side of the NJR4556. This would give an impedance of 16 ohms so that looks feasible. So I will be replacing the NE5532 in the balanced transmitter with an NJR4556 to drive either the amp or headphones.

while the 4556 is not that well regarded as an audio device, the measurements show it is well suited to driving headphones and is more that good enough to use to both drive the amp/headphones and the DI out.  

I recently used a head that (to my surprise) did not mute the speaker output when the headphones were plugged in. I queried it with the manufacturer and was told that they considered that a small lightweight amp would be used without a cabinet with headphones so it was no necessary. Although I originally disagreed, I have to admit there is some merit to the argument. 

Disconnecting the speaker in a combo is not an option so the speakers must be muted. Luckily I can use a double pole double throw switch to both ground the enable pin on the icepower module (muting the power amp) and divert the signal from the amp input to the headphone jack. 

[Chienmortbb]

 

So I have finally finished the amp section of the After Eighty.  I have connected the headphones only when the mute/standby switch is activated. The headphone side needs more gain as if you disconnect the headphones, the speaker output is so much louder. However the sound of the amp (through my 8" PA Cab) is really nice, I can't wait to try it with its own cabinet.

Now it is time to sort out the cosmetics. The side of the cab where the port is needs sanding and repainting and I need to try to make some decals to reflect the new functions I have added.

 

 

[Chienmortbb]

I have been in electronics since I left School and have done a smaall amount of coding/programming professionally, The thing I should always remember is that a good knight's sleep often gives you the answer.

I forgot that the outputs of the opamps in the circuit above have 100 ohm resistors in line. With my headphones being connected in parallel and having a combined impedance of 16* ohms, the output will be attenuated by 15-16dB (* the exact amount depends on the impedance of the heaphones at a particulat frequency). So by taking the output directly from the opamp output, the headphones will recieve more power. Unfortunately this means soldering a wire directly onto the PCB but as much of the preamp is hardwired, it is  not a massive problem.

[Mottlefeeder]

5 hours ago, Chienmortbb said:

I have been in electronics since I left School and have done a smaall amount of coding/programming professionally, The thing I should always remember is that a good knight's sleep often gives you the answer.

I forgot that the outputs of the opamps in the circuit above have 100 ohm resistors in line. With my headphones being connected in parallel and having a combined impedance of 16* ohms, the output will be attenuated by 15-16dB (* the exact amount depends on the impedance of the heaphones at a particulat frequency). So by taking the output directly from the opamp output, the headphones will recieve more power. Unfortunately this means soldering a wire directly onto the PCB but as much of the preamp is hardwired, it is  not a massive problem.

Rod Elliott recommends using the 100 ohm resistors on the outputs to ensure stability, especially when connecting cables to those outputs, so if you dispense with the resistors it might be advisable to put a small capacitor in parallel with the feedback resistor to roll off the gain at high frequencies.

David

Edited December 23, 2020 by Mottlefeeder

[Chienmortbb]

On 23/12/2020 at 18:00, Mottlefeeder said:

Rod Elliott recommends using the 100 ohm resistors on the outputs to ensure stability, especially when connecting cables to those outputs, so if you dispense with the resistors it might be advisable to put a small capacitor in parallel with the feedback resistor to roll off the gain at high frequencies.

David

I have changed the 100R resistors i  the DI output to 220R and added the Zeners as per Rod Elliotts bass amp design.

 

The whole amp is now together but I need to reduce the gain of the AUX input somewhat as it over powers the bass. I need to investigate further as the volume seems a little low. I will get  the oscilloscope out tomorrow and check the levels.

I have ordered the waterslide decal paper so that I can label the added features. If I cannot get a good match to the existing text colour, I may respray the control panel to do it.   

[Passinwind]

Great to see more progress John. The EQ test plots you sent me look really good, and pretty much just as expected.

[Chienmortbb]

As @Passinwind mentions, I did some frequency response plots of the preamp at extremes of the EQ settings. Here they are:

Flat all EQ set to 12 o'clock.EQ at

The small dip at 2KHz is only 0.5dB and would not be enough to hear. ON the later revision of the pre-amp, there was an optional fully Parametric Equaliser PEQ combined with an High Pass Filter. The PEQ could be used to take out the 2KHz dip if required. I am using those two circuits on mmy self build amp head that will be finished once I have finished this build,

Bass Control Fully Up

 

Bass Control Fully Down

 

 

 

 

Middle Fully Up

 

 

Middle Fully Down

 

Treble Fully Up

Treble Fully Down

[Chienmortbb]

As you can see I have been playing with my Picoscope PC oscilloscope. The frequency response plots are from some software designed by another Picoscope user. The Picoscope oscilloscope has an arbitrary waveform generator built in as well as the capability to do  many other measurements.

The output of the preamp with a 1V input signal  is 6.73V RMS with a +/-15V rail. That should be enough to fully drive the 50ASX2 power amp although I have yet to measure the output of my bass.

The headphone/mute switch works well and the output of the 4556 op-amps is more than enough to drive my headphones.

The waterslide decals did not perform as well as hoped. For my partially finished head amp, I used laser waterslide decals for the legend and it worked really well. Sadly the laser printer is not working so I had to use the inkjet.  Well the inkjet decals were thinnner and did not take the colour well. I have decided to just spray over legend that is cleary now wrong and leave the rest unlabled. The reason is that I only really intended to build a better practice amp with availble parts. Whilst I had to order a few things, as it stands I have achieved that and it would take a lot of work to take it to the next level.

The amp sound great. It is a very neautral sound, not sterile as all. On the floor there is plenty of low end and enough control to get a really nice sound using my Aerodyne Bass.

All that now remains is a touch up of the tuffcab where the port was installed and for the grille to be installed.

Please accept my apologies for the quality of the images and the fact that I inadvertantly selected video rather than photos for some.

 

Edited January 11, 2021 by Chienmortbb

[Chienmortbb]

I ma amazed that it is almost 5 months since I posted and since then I have been playing through the amp every day. I have a few observations on tge amp and some mods that I will be doing. After some discussion with @Passinwind who designed the pre-amp, I will be modifying the treble control. Although I rarely boost treble and can cut it on the Tone pot of the bass, it seems odd to have a control that seems to do nothing.

Secondly, I always needed to add an HPF as the speaker can only go so low and it is a waste of energy and headroom to let those frequencies through. The plan is for a fixed frequency 24dB/octave with a -3dB frequency of 50Hz. This may seem high but it has little effect on the real frequency response of the cab and means that at 100W, the cone excursion is well within the speakers capabilities down to a few Hz.

Finally the power amp is capable of 100W into 8 ohms and 170 Watts into 4 ohms. It's just asking for an extension cabinet. So a Fane 8-225 speaker will be fitted into a matching box to utilise the whole power of the amp. So  2x8 stack fed with170 watts, ideal for my open mic gigs IF they start up again.

Finally (for this post) a big thank for their help in this project you to @stevie, @Phil Starr and to @Passinwind

[Passinwind]

1 hour ago, Chienmortbb said:

I ma amazed that it is almost 5 months since I posted and since then I have been playing through the amp every day. I have a few observations on tge amp and some mods that I will be doing. After some discussion with @Passinwind who designed the pre-amp, I will be modifying the treble control. Although I rarely boost treble and can cut it on the Tone pot of the bass, it seems odd to have a control that seems to do nothing.

 

Heh, "I told you so" comes to mind about now!" 😎

That circuit was designed for use with high quality tweeters not turned down and an active bass that handles the high midrange, and doesn't do much at all in either boost or cut until you get to 4KHz. Turned up it's essentially a classic constant directivity horn EQ, all about getting high frequency content out to the walls. I used to do DIY onboard preamps like that too (the the ampp needs to do high mods), and still have one like that in one fretless that's set up for slide bass duty. For me that gives more or less zero hiss even with full boost , and I do tend to boost treble a bit on almost everything as all my instruments use humbuckers. When I started doing onboard electronics for actual paying customers I worked out a different tradeoff set for them, which is what we'll be trying here, but the vast majority of what I've sold just uses a more conventional treble circuit and live with a little hiss at extreme boost settings.

Anyhow, things are looking good John. Looking forward to seeing how the bigger amp comes out with the new case too.

Edited April 30, 2021 by Passinwind