Sunday, 4 December 2022

The GAA-12 Amp's little Sister — "Suzu" & 10 inch Speaker Choice

In this blog post, I'll update you on the smaller size version of the GAA -12 Practice Guitar Amp which we call "Suzu" and the speaker and cabinet arranged for both amps.

1. Ten inch Speaker

I chose the Eminence Legend 1058 speaker for both practice amp speakers. My vibe = a Fender 1960s guitar sound playing Telecasters with a single 10 inch speaker. My preamplifier is designed around that speaker size and that guitar.

Fortunately, many kind YouTube posters have uploaded head-to-head trials with various 10 inch guitar speakers for comparison. I tend to favour Alnico magnet 10 inch speakers, however, dislike their cost. My "non Alnico" preference seemed to the the Legend 1058 in several videos. So I bought one and found it well suited my purposes. — and the added bonus,  it's not expensive.

Above — The large dust cap makes the speaker look bigger than 10 inches in diameter. This speaker is a gem. Ferrite magnet and weighs 2 Kg.

Above — Transfer function of the Legend 1058 from Eminence. It directly connects to what I hear with actual playing tests. In a cube shaped cabinet with my preamp circuit, the bass + lower middle response seems acceptably moderate + reasonably flat. There is 1 "sharp" peak at ~2700 Hz, but the treble response starts to fall down a cliff at around 5 KHz. Perfect for a Fender Telecaster through a 10 inch speaker.

Above — My wife designed & built a prototype cabinet from a plank of 12 inch wide, 3/4 inch thick pine. The final specs are 12 inches depth x 12 inches height x 14 inches width  [ or 30.48 cm deep x 30.48 cm height X 35.56 cm width ]. I stuffed some fibreglass pink insulation in the cavity to dampen any reflecting waves. The back is partially open with a 2 inch gap across the top end. This keeps out cats (protects the speaker), keeps in the insulation and gives punchy bass tones with some room audio fill through he back of the speaker cabinet.

Above — Testing the prototype cabinet. At this point, the feet, handles, grill and corner bumpers are not installed. Smooth, punchy bass -- and a crisp, but not ear-piercing high frequency response. Sounds lovely --- that's a wrap. The cabinet is small plus light.

2. Amplifier News

Above — For the first time ever, I'm using a commercial grade bridge rectifier and will also use this part in my high powered amps. You may heat sink the GBUE2560 as well.

Above — Rectifier and reservoir caps for the DC power supply.

Above — The power supply transformer just sitting in the chassis prior to wire shortening and mounting.The Hammond 166L25 gives 12 watts out, while the166L20 gives about 8 watts clean output power. Further, if you regulate the op-amp supply with the 166L20, this means running +/-12 volts split as the unregulated DC voltage sags downs to less than 14 VDC on each rail when driven hard.

I also tested a larger transformer with 29 VDC unregulated on each rail  and for awhile, Suzu was running at 27 Watts output power. The Hammond 166L25 and 166L20 have identical dimensions. In the end, I opted with the 166L25, since its higher output DC voltage allows you to run the op-amp rails at 16 volts regulated to get maximum headroom.

Above — The power supply section mounted and tested.

Above — PA schematic. I chose different transistors for the input emitter coupled pair and also for the finals. Further, I sank a little more current in the emitter coupled pair and the VAS/driver stack. At this point, I only plan to run voltage feedback in the global feedback loop, although, I can easily add current feedback if desired.

I measured a β of 540 for BC546B matched pair. The whole BC54-X- series seem an incredible BJT collection offering  low noise figure plus high β and, of course, is long obsolete. I've got 30 pieces of the über low NF BC549 in my parts bins for future 12 volt single-supply, discrete, low-noise AF amplifiers.

Above — Notice from Mouser. The day after I installed the power Darlington complimentary finals, I got this notice by email. Obsolescence might be the central story of my electronic hobbyist career ? Happily, I've got enough genuine power follower pairs -- both standard and Darlington style to last me for a long time.

Above —The finals mounted in their heat sinks. Once again a hack saw helped fashion DYI heat sinks.

Above — The finals and PA mounted in the "cake pan". The power transformer was unmounted in this photo. Suzu has a smaller chassis and  will go upstairs in our living room as my main practice amp. The downstairs GAA -12 amp serves as my main transcription amplifier. I spend time downstairs  transcribing horn solos. I rarely, if ever listen to guitarists other than if a guitar happened to be on the song of the horn player whom I'm transcribing. 

Above — Suzu's PA offers low distortion. I'm very happy with this PA stage. The matched input pair have obliterated the 2nd harmonic and I believe what's left are crossover + some intermodulation products from interactions with my outboard circuit, test leads, clips and probes.

Above — I've got a small single op-amp preamp board taped on the front corner during my initial testing of the PA. It's also interesting to play guitar through a preamp with no tone control section. You hear lots of mids and also muddly lows around 200 Hz or so. Suzu will have 1 volume control and then 3 other pots for tone alteration. I may add a small front panel switch if needed. Small and simple.

Above —25K linear taper tone control pots on the left. I also ordered a few audio taper 10K pots ( for volume control ) and for the first time ever ---  pots with a center detent (25K linear). I'm making my tone control circuits with a maximum value of 25K on the potentiometers these days.

I'm now getting ready to work on the preamp. I'm about 85-90% happy with the current preamp on the GAA -12 Practice Amp. After a new single coil pickup arrives -- and with my speaker + cabinet chosen/built, I can spend much time to try to nail the tone I seek.

Best regards!   Click here for my Guitar-Related Index


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